The conjugate of mitomycin C (MMC) with carboxymethylated schizophyllan (CMSPG) which was prepared from monochloroacetic acid and schizophyllan (SPG), a beta-(1-->6)-branched (1-->3)-beta-D-glucan from Schizophyllum commune Fries, was synthesized by using 1-ethyl-3-(3-dimethylaminopropyl)- carbodiimide. The degree of the substitution of carboxymethyl groups in CMSPG was estimated as approximately 0.87, and locations of carboxymethyl groups in CMSPG were predominantly determined at O-4, O-6, and O-4, 6 positions in glucose residues. The contents of MMC in the conjugate were estimated to be between 8 and 12% (w/w). The conjugate showed successive monoexponential liberation, with a half-life of 7.2 h. Although the in vitro cytotoxicity of the conjugate against L1210 leukemia cells was similar to that of MMC when the cells were exposed for 24 and 48 h, the 50% growth-inhibitory concentration of the conjugate for L1210 was two times higher than that of MMC with exposure for 12 h. The antitumor activity of the conjugate against subcutaneously implanted sarcoma 180 solid tumor in mice by intraperitoneal (i.p.) administration was similar to that of MMC at a dose of 1.5 mg eq MMC per kg per d for both 7 times of continuous administration and 4 times of intermittent administration. However, the reduction in the number of leukocytes in the peripheral blood, which was the side effect of MMC, was suppressed by the intermittent administration of the conjugate. The conjugate maintained the ability to induce the tumor regressing factor and the neutrophil chemotactic factor in the serum.