Mitomycin C-Dextran conjugates (MMC-D) were intravenously (iv) injected in mice bearing subcutaneous sarcoma 180. The tissue distribution was determined for three 14-C-labeled anionic conjugates (MMC-Dan) with molecular weights of 10, 70, and 500 kd and one cationic 70-kd 14C-conjugate (MMC-Dcat). The anionic conjugates were slowly cleared from the plasma, and their elimination rate decreased with increasing molecular weight. Radioactivity accumulated in liver, spleen, lymph nodes, and tumor but not in heart, lung, intestines, kidney, or muscle after iv injection of all types of 14C-MMC-Dan. In contrast, the cationic conjugate was rapidly cleared from the plasma and accumulated mostly in the liver and spleen, while tumor levels remained low. The antitumor effect of the 70-kd MMC-Dan, which afforded the highest tumor concentration, was superior to that of free MMC. Therefore, anionic mitomycin C-dextran conjugates with a high molecular weight may be useful for tumor targeting in cancer chemotherapy.