A heterobifunctional reagent, N-succinimidyl 3-(2-pyridylidithio)propionate, was synthesized and characterized. This reagent contains an N-hydroxysuccinimide ester group that reacts with amino groups and a 2-pyridyl disulphide group that reacts with aliphatic thiols. It was used to introduce thiol groups into proteins through a two-step process: first, the 2-pyridyl disulphide structures are introduced into the protein by reacting amino groups with the ester group of the reagent. Then, the protein-bound 2-pyridyl disulphide structures are reduced with dithiothreitol without affecting native disulphide bonds. This method was applied to proteins such as ribonuclease, γ-globulin, α-amylase, and horseradish peroxidase. The reagent can also be used to form protein-protein conjugates by reacting thiol-containing proteins with protein-2-pyridyl disulphide derivatives via thiol-disulphide exchange. This conjugation technique was used to prepare conjugates such as α-amylase-urease, ribonuclease-albumin, and peroxidase-rabbit anti-human transferrin antibody. The disulphide bridges between the proteins can be reversed by reduction or thiol-disulphide exchange, making the conjugation reversible. The reagent was synthesized in crystalline form and stored at room temperature. It was shown to be effective for introducing aliphatic thiols into amino group-containing molecules. The reagent was also used to prepare various protein conjugates, including ribonuclease-mercaptalbumin, α-amylase-urease, and peroxidase-rabbit anti-human transferrin antibody. The conjugates were analyzed by gel filtration and showed distinct molecular weights, indicating covalent bonding. The reagent was found to be stable under various conditions and could be used for the preparation of enzyme-labeled antibodies and antigens for immunoassays. The method allows for the preparation of conjugates with different sizes, including bimolecular, termolecular, and polymeric conjugates. The reagent was also used to demonstrate the reversible nature of protein-protein conjugation. The results showed that the conjugates could be separated into their components by reduction, confirming the reversibility of the conjugation process. The reagent was found to be effective for introducing thiol groups into proteins and for forming protein-protein conjugates. The method was applied to various proteins and showed promising results for the preparation of conjugates with different molecular weights and structures. The reagent was also used to demonstrate the possibility of preparing oligomeric heteroconjugates. The conjugates were analyzed by gel filtration and showed distinct molecular weights, indicating covalent bondingA heterobifunctional reagent, N-succinimidyl 3-(2-pyridylidithio)propionate, was synthesized and characterized. This reagent contains an N-hydroxysuccinimide ester group that reacts with amino groups and a 2-pyridyl disulphide group that reacts with aliphatic thiols. It was used to introduce thiol groups into proteins through a two-step process: first, the 2-pyridyl disulphide structures are introduced into the protein by reacting amino groups with the ester group of the reagent. Then, the protein-bound 2-pyridyl disulphide structures are reduced with dithiothreitol without affecting native disulphide bonds. This method was applied to proteins such as ribonuclease, γ-globulin, α-amylase, and horseradish peroxidase. The reagent can also be used to form protein-protein conjugates by reacting thiol-containing proteins with protein-2-pyridyl disulphide derivatives via thiol-disulphide exchange. This conjugation technique was used to prepare conjugates such as α-amylase-urease, ribonuclease-albumin, and peroxidase-rabbit anti-human transferrin antibody. The disulphide bridges between the proteins can be reversed by reduction or thiol-disulphide exchange, making the conjugation reversible. The reagent was synthesized in crystalline form and stored at room temperature. It was shown to be effective for introducing aliphatic thiols into amino group-containing molecules. The reagent was also used to prepare various protein conjugates, including ribonuclease-mercaptalbumin, α-amylase-urease, and peroxidase-rabbit anti-human transferrin antibody. The conjugates were analyzed by gel filtration and showed distinct molecular weights, indicating covalent bonding. The reagent was found to be stable under various conditions and could be used for the preparation of enzyme-labeled antibodies and antigens for immunoassays. The method allows for the preparation of conjugates with different sizes, including bimolecular, termolecular, and polymeric conjugates. The reagent was also used to demonstrate the reversible nature of protein-protein conjugation. The results showed that the conjugates could be separated into their components by reduction, confirming the reversibility of the conjugation process. The reagent was found to be effective for introducing thiol groups into proteins and for forming protein-protein conjugates. The method was applied to various proteins and showed promising results for the preparation of conjugates with different molecular weights and structures. The reagent was also used to demonstrate the possibility of preparing oligomeric heteroconjugates. The conjugates were analyzed by gel filtration and showed distinct molecular weights, indicating covalent bonding