A human monocyte-derived neutrophil chemotactic factor (MDNCF) was purified and characterized. This protein, which is chemotactic for human neutrophils, was isolated from the conditioned medium of lipopolysaccharide (LPS)-stimulated monocytes through a series of purification steps including anion-exchange chromatography, gel filtration, and high-performance liquid chromatography (HPLC). The purified protein migrated as a single 7-kDa band on NaDodSO4/polyacrylamide gels under both reducing and nonreducing conditions. The amino acid composition of MDNCF was different from that of interleukin 1 (IL-1) and tumor necrosis factor (TNF). The N-terminal amino acid sequence of MDNCF showed up to 56% sequence similarity with several host defense cytokines. MDNCF was found to be identical to a portion of a sequence deduced from an mRNA induced by staphylococcal enterotoxin treatment of human leukocytes. At an optimal concentration of 10 nM, 50% of neutrophils added to chemotaxis assay wells migrated toward the pure attractant. MDNCF's potency and efficacy were comparable to that of fMet-Leu-Phe, a commonly used reference. Unlike many attractants, MDNCF was not chemotactic for human monocytes. MDNCF is potentially a mediator of a leukocyte-specific inflammatory response, as it is released by an inflammatory stimulus and has the selective capacity to attract neutrophils but not monocytes. The study also showed that MDNCF is structurally and functionally related to other proteins involved in inflammation and immune regulation, including platelet factor 4, β-thromboglobulin, and γIP-10. The results suggest that MDNCF may play a role in host defense by attracting neutrophils to sites of inflammation.A human monocyte-derived neutrophil chemotactic factor (MDNCF) was purified and characterized. This protein, which is chemotactic for human neutrophils, was isolated from the conditioned medium of lipopolysaccharide (LPS)-stimulated monocytes through a series of purification steps including anion-exchange chromatography, gel filtration, and high-performance liquid chromatography (HPLC). The purified protein migrated as a single 7-kDa band on NaDodSO4/polyacrylamide gels under both reducing and nonreducing conditions. The amino acid composition of MDNCF was different from that of interleukin 1 (IL-1) and tumor necrosis factor (TNF). The N-terminal amino acid sequence of MDNCF showed up to 56% sequence similarity with several host defense cytokines. MDNCF was found to be identical to a portion of a sequence deduced from an mRNA induced by staphylococcal enterotoxin treatment of human leukocytes. At an optimal concentration of 10 nM, 50% of neutrophils added to chemotaxis assay wells migrated toward the pure attractant. MDNCF's potency and efficacy were comparable to that of fMet-Leu-Phe, a commonly used reference. Unlike many attractants, MDNCF was not chemotactic for human monocytes. MDNCF is potentially a mediator of a leukocyte-specific inflammatory response, as it is released by an inflammatory stimulus and has the selective capacity to attract neutrophils but not monocytes. The study also showed that MDNCF is structurally and functionally related to other proteins involved in inflammation and immune regulation, including platelet factor 4, β-thromboglobulin, and γIP-10. The results suggest that MDNCF may play a role in host defense by attracting neutrophils to sites of inflammation.