The paper introduces a new heterobifunctional reagent, N-succinimidyl 3-(2-pyridyldithio)propionate, which can be used for protein thiolation and reversible protein-protein conjugation. The reagent consists of an N-hydroxysuccinimide ester group that reacts with amino groups and a 2-pyridyl disulphide structure that reacts with aliphatic thiols. The method involves two steps: first, 2-pyridyl disulphide structures are introduced into the protein by reacting its amino groups with the N-hydroxysuccinimide ester side of the reagent, and then the protein-bound 2-pyridyl disulphide structures are reduced with dithiothreitol. This technique has been successfully applied to introduce thiol groups into ribonuclease, γ-globulin, α-amylase, and horseradish peroxidase. Additionally, the reagent can be used to prepare protein-protein conjugates by reacting protein-2-pyridyl disulphide derivatives with thiol-containing proteins via thiol-disulphide exchange. The disulphide bridges between protein molecules can be easily cleaved by reduction or thiol-disulphide exchange, making the conjugation reversible. The reagent is stable in crystalline form and can be stored at room temperature. The paper also discusses the synthesis and properties of the reagent, as well as the methods for introducing 2-pyridyl disulphide structures into proteins and preparing protein-protein conjugates.The paper introduces a new heterobifunctional reagent, N-succinimidyl 3-(2-pyridyldithio)propionate, which can be used for protein thiolation and reversible protein-protein conjugation. The reagent consists of an N-hydroxysuccinimide ester group that reacts with amino groups and a 2-pyridyl disulphide structure that reacts with aliphatic thiols. The method involves two steps: first, 2-pyridyl disulphide structures are introduced into the protein by reacting its amino groups with the N-hydroxysuccinimide ester side of the reagent, and then the protein-bound 2-pyridyl disulphide structures are reduced with dithiothreitol. This technique has been successfully applied to introduce thiol groups into ribonuclease, γ-globulin, α-amylase, and horseradish peroxidase. Additionally, the reagent can be used to prepare protein-protein conjugates by reacting protein-2-pyridyl disulphide derivatives with thiol-containing proteins via thiol-disulphide exchange. The disulphide bridges between protein molecules can be easily cleaved by reduction or thiol-disulphide exchange, making the conjugation reversible. The reagent is stable in crystalline form and can be stored at room temperature. The paper also discusses the synthesis and properties of the reagent, as well as the methods for introducing 2-pyridyl disulphide structures into proteins and preparing protein-protein conjugates.