Abstract: Antibiotic C1027, a macromolecular peptide with highly potent cytotoxicity to cultured cancer cells, was conjugated to monoclonal antibody 3A₅ and its Fab fragment separately using SPDP as the linker agent. McAb 3A₅, identified as IgGl, was directed against human hepatoma BEL--7402 cells. By ELISA, McAb 3A₅ and the Fab fragment were strongly reactive with hepatoma cells and weakly reactive with KB cells. Determined by clonogenic assay against hepatoma BEL--7402 cells, the IC₅₀ values for McAb--C1027, Fab--C1027 and C1027 were 4.2×10^-14, 8. 6×10^-16 and 6.5×10^-16 mol· L^-1, respectively. Fab--C1027 was 49--fold more potent than McAb--C1027 in cytotoxicity. Moreover, Fab--C1027 was 160--fold more potent in cytotoxicity to hepatoma cells than to KB cells, indicating selective cytotoxicity of Fab--C1027 conjugate to the target cells. Therapeutic effect of the conjugate was evaluated with hepatoma BEL--7402 xenograft in nude mice. After subcutaneous transplantation of the tumor, treatment started on day 3, ⅳ, with equivalent dose of C1027, 0. 1 mg · kg^-1×6. Fab--C1027 and free C1027 inhibited tumor growth by 85% and 59%, respectively. Fab--C1027 conjugate showed more marked antitumor effect in vivo than free C1027.