Efficacy of antibody mediated targeting depends on retention of immunoreactivity in conjugates. Retention can be improved by site-specific linkage of drugs or drug-loaded carriers to residues that are located well away from the antigen-binding sites. In this study we describe the site-specific linkage of a potential drug carrier, human serum albumin (HSA), to the carbohydrate residues in Dal K20, a murine IgG1 monoclonal antibody (mAb) against human renal cell carcinoma, using disulfide exchange between 3-(2-pyridyldithio)propionic acid succinimidyl ester (SPDP)-derivatized HSA and 11-[[3-(2-pyridyldithio)propionyl]amino]undecanoic acid hydrazide (AUPDP)-derivatized mAb Dal K20. AUPDP gave a higher yield of the conjugate than a functionally analogous 3-(2-pyridyldithio)propionic acid hydrazide (HPDP), suggesting that the extra length of the former facilitated the linkage. The conjugates were found to be unstable without reduction of the hydrazone linkage using sodium cyanoborohydride. Stabilized 1:1 HSA:K20 carbohydrate-linked conjugates were isolated and compared with non-site-specific 1:1 conjugates in which HSA was conjugated to amino groups in mAb Dal K20. The yield and stability of the two conjugates were comparable, but the site-specific conjugate was found to retain three times more antibody activity than the non-site-specific conjugate.