Abstract: The clinic use of streptonigrin (114B), a highly active antitumor antibi-otic, is limited by its detrimental effects on normal tissues. In an attempt to improve itsspecificity streptonigrin was conjugated to anti-human hepatoma monocloal antibody3A5 by four different chemical linkage methods. The first method was via water-solubecarbodiimide (EDCI) to create conjugates (1); in the second, an active ester ofstreptonigrin was applied as a reactive intermediate (2); and in the other two, spacerswere put to use for coupling streptonigrin to McAb 3A5-Dextran T-40(3) or McAb3A5-bovin senrnm albumin (BSA) (4). The conjugates showed biological activities andUV spectra characteristics of streptonigrin and 3A5. As determined by clonogenic assaywith 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, respec-tively. The potency of conjugates(2) and (3) were 63-fold and 18-fold stronger thanthat of free streptonigrin. Clonogecic assay with KB cells which weakly react with 3A5 byElisa showed that the potency of conjugate (2) and (3) were 11-fold and 13-foldweaker than free streptonigrin, respectively. The results suggest that the conjugates ofMcAb 3A5 and streptonigrin show specific cytotoxicity to target liver cancer cails. Thelinkage groups of streptonigrin were also discussed.