RPSP, a refined polysaccharide peptide fraction isolated by fast performance liquid chromatography (FPLC) from the crude powder of total peptide-bound polysaccharides of cultivated Coriolus versicolor Cov-1 dose-dependently inhibited the proliferation of a human hepatoma cell line (HEPG2). The effective dose causing 50% inhibition following 3-day exposure to RPSP was 243 +/- 36 micrograms/ml for HEPG2. However, little or no inhibitory effects were detected in normal human foetal hepatocytes. On the other hand, in the pretreatment group, in which RPSP was administered i.p. for two weeks before sarcoma 180 inoculation in nude mice, the incidence of tumor growth was less (2 out of 5 mice) than that of the control group (all 5 mice). The tumor size of the control group was about 3-5 times bigger than that of the pretreatment group. In tumor-bearing nude mice, 5 days after sarcoma 180 inoculation, i.v. administration of RPSP significantly suppressed the growth of tumor mass. The inhibition rate was 93.6% on day 13. Furthermore, administration of RPSP did not cause any pathological lesions in vital organs of rabbits such as heart, liver, spleen, lung and kidney. In conclusion, these results indicate that RPSP acts by directly suppressing tumor cell growth in vitro and the prevention of in vivo growth of tumor mass is probably mediated also via its immunomodulating effects.