1. The antitumour effect of orally administered cordycepin, a component isolated from water extracts of Cordyceps sinensis, was examined in mice inoculated with B16 melanoma (B16-BL6) cells. 2. B16-BL6 (1 x 10(6)) cells were inoculated subcutaneously into the right footpad of mice. At 2 weeks after the cell inoculation, the enlarged primary tumour lump was weighed. Cordycepin (0, 5 and 15 mg/kg per day) was administered orally to the mice for 2 weeks from the date of tumour inoculation. Cordycepin (15 mg/kg per day) significantly reduced by 36% the wet weight of the primary tumour lump compared to that of the untreated control mice, without any loss of bodyweight or systemic toxicity. 3. Cordycepin (15 mg/kg per day) administered orally for 2 weeks inhibited the tumour enlargement in the right thigh inoculated with B16-BL6 cells premixed with extracellular matrix (Matrigel). 4. These results indicate that orally administered cordycepin inhibits melanoma cell growth in mice with no adverse effects.