In this study, an attempt was made to elucidate the combined effect of 2'-deoxycoformycin (DCF), an adenosine deaminase inhibitor, with a water extract of Cordyceps sinensis (WECS), on the growth curves of mouse melanoma and lung carcinoma cells. Sub-confluent cells were harvested with an EDTA trypsin solution, and resuspended to appropriate concentrations in DMEM containing 10% fetal bovine serum. Using 1x10(5) cells/2 ml in each well of a 12-well culture plate, cells were incubated for 24, 48 and 72 h in the presence of WECS alone, or WECS plus DCF in a CO2 incubator at 37 degrees C. Duplicate samples of viable cells were enumerated with a Coulter counter. The antitumor effect of WECS on the growth curves of tumor cell lines increased over 3-fold in combination with DCF. These results suggest that DCF has a remarkable reinforcement effect on the antitumor activity of WECS. DCF is a potent adjuvant for WECS.