The clinic use of streptonigrin (114B), a highly active antitumor antibiotic, is limited by its detrimental effects on normal tissues. In an attempt to improve its specificity streptonigrin was conjugated to anti-human hepatoma monoclonal antibody 3A5 by four different chemical linkage methods. The first method was via water-soluble carbodiimide (EDCI) to create conjugates (1); in the second, an active ester of streptonigrin was applied as a reactive intermediate (2); and in the other two, spacers were put to use for coupling streptonigrin to McAb 3A5-Dextran T-40 (3) or McAb 3A5-bovin serum albumin (BSA) (4). The conjugates showed biological activities and UV spectral characteristics of streptonigrin and 3A5. As determined by clonogenic assay with human hepatoma BEL-7402 cells for 1 hour exposure, the IC50 for conjugate (2), conjugate (3) and streptonigrin were 0.355 ng/ml, 1.23 ng/ml and 22.4 ng/ml, respectively. The potency of conjugates (2) and (3) were 63-fold and 18-fold stronger than that of free streptonigrin. Clonogenic assay with KB cells which weakly react with 3A5 by Elisa showed that the potency of conjugate (2) and (3) were 11-fold and 13-fold weaker than free streptonigrin, respectively. The results suggest that the conjugates of McAb 3A5 and streptonigrin show specific cytotoxicity to target liver cancer cails. The linkage groups of streptonigrin were also discussed.