A conjugate of bleomycin (BLM) and the mannan of bakers' yeast (Saccharomyces cerevisiae wild type strain) (WNM) was synthesized. The assay of its antimetastatic effect on Lewis lung carcinoma (3LL) implanted in C57BL/6 mice showed that this conjugate exhibited a higher antimetastatic effect and longer life-span elongation than those of free bleomycin and mannan with corresponding doses. This conjugate was also found to kill the 3LL cells in vitro. 14C-Labeled mannan-bleomycin conjugate was much more bound that 14C-labeled dextran-bleomycin conjugate to the 3LL. It was concluded that the anti-cancer mechanism of this conjugate, WNM-BLM possessed a specific binding effect to the tumor cells and exhibited a cytocidal effect on the 3LL target cells.