A method is presented for the isolation of gene sequences by using antibody probes. A λ phage expression vector, λ gt11, has been constructed whose properties permit insertion of foreign DNA to produce a recombinant DNA library, high-frequency lysogeny in particular Escherichia coli strains, induced synthesis of β-galactosidase fused to protein specified by the foreign DNA, and reproducible detection of antigen in populations of up to 10⁶ lysogens per 82-mm nitrocellulose filter. Other features and potential uses of the vector are described. The λ gt11 vector is a phage expression vector that allows the insertion of foreign DNA into the β-galactosidase structural gene lacZ, promoting the synthesis of hybrid proteins. It enables efficient screening of antigen-producing clones in λ gt11 recombinant cDNA libraries through lysogeny of the phage library in hflA (high-frequency lysogeny) mutant cells of Escherichia coli. Lysogens produce detectable quantities of antigen on induction, even when plated at high cell densities. The vector is also designed to facilitate the isolation of proteins specified by previously cloned gene sequences. Hybrid proteins encoded by recombinant phage accumulate in strains defective in protein degradation (lon mutants) in amounts amenable to large-scale purification. Antibodies produced against the portion of the hybrid encoded by foreign DNA could in turn be used to isolate the native polypeptide from eukaryotic cells. The λ gt11 vector has been constructed to allow the construction of cDNA libraries of 10⁵–10⁷ recombinants from which individual antigen-producing clones can be isolated efficiently. The vector has properties that permit the construction and maintenance of large cDNA libraries. The recombinant should be propagated in its host cell as a single-copy genomic insert to enhance its stability and to facilitate repression of foreign genetic information. The expression vector should also respond to induction with a rapid increase in copy number and high-level transcription of the foreign DNA. Because the ability to detect antigen will depend on its stability, the expression vector and its host should include features that minimize the degradation of the foreign eukaryotic protein. The properties of the λ gt11 vector include the general ability of λ lysogens to produce large quantities of phage products on induction, which has been exploited to enhance the sensitivity and efficiency of antigen screening. The construction of a library of recombinant DNA-containing lysogens with λ gt11 permits the growth, induction, and lysis of antigen-producing cells directly on nitrocellulose filters. The vector has been used to isolate protein-encoding genes from large recombinant DNA libraries by using antibodies to detect antigen produced by specific recombinants. The vector has also been used to isolate unknown proteins encoded by cloned DNA. The hybrid proteins produced by the vector are stable in prokaryotic cellsA method is presented for the isolation of gene sequences by using antibody probes. A λ phage expression vector, λ gt11, has been constructed whose properties permit insertion of foreign DNA to produce a recombinant DNA library, high-frequency lysogeny in particular Escherichia coli strains, induced synthesis of β-galactosidase fused to protein specified by the foreign DNA, and reproducible detection of antigen in populations of up to 10⁶ lysogens per 82-mm nitrocellulose filter. Other features and potential uses of the vector are described. The λ gt11 vector is a phage expression vector that allows the insertion of foreign DNA into the β-galactosidase structural gene lacZ, promoting the synthesis of hybrid proteins. It enables efficient screening of antigen-producing clones in λ gt11 recombinant cDNA libraries through lysogeny of the phage library in hflA (high-frequency lysogeny) mutant cells of Escherichia coli. Lysogens produce detectable quantities of antigen on induction, even when plated at high cell densities. The vector is also designed to facilitate the isolation of proteins specified by previously cloned gene sequences. Hybrid proteins encoded by recombinant phage accumulate in strains defective in protein degradation (lon mutants) in amounts amenable to large-scale purification. Antibodies produced against the portion of the hybrid encoded by foreign DNA could in turn be used to isolate the native polypeptide from eukaryotic cells. The λ gt11 vector has been constructed to allow the construction of cDNA libraries of 10⁵–10⁷ recombinants from which individual antigen-producing clones can be isolated efficiently. The vector has properties that permit the construction and maintenance of large cDNA libraries. The recombinant should be propagated in its host cell as a single-copy genomic insert to enhance its stability and to facilitate repression of foreign genetic information. The expression vector should also respond to induction with a rapid increase in copy number and high-level transcription of the foreign DNA. Because the ability to detect antigen will depend on its stability, the expression vector and its host should include features that minimize the degradation of the foreign eukaryotic protein. The properties of the λ gt11 vector include the general ability of λ lysogens to produce large quantities of phage products on induction, which has been exploited to enhance the sensitivity and efficiency of antigen screening. The construction of a library of recombinant DNA-containing lysogens with λ gt11 permits the growth, induction, and lysis of antigen-producing cells directly on nitrocellulose filters. The vector has been used to isolate protein-encoding genes from large recombinant DNA libraries by using antibodies to detect antigen produced by specific recombinants. The vector has also been used to isolate unknown proteins encoded by cloned DNA. The hybrid proteins produced by the vector are stable in prokaryotic cells