The sensitivity of Saccharomyces cerevisiae to the antibiotic cordycepin (3'-deoxyadenosine) was found to be decreased by the addition of thiamin to the growth medium. A thiamin transport mutant of S. cerevisiae was also found to be resistant to the growth inhibitory effect of cordycepin. Not only the thiamin uptake but also adenosine uptake by this mutant cell was markedly reduced compared to those by the parent yeast cells. This strongly suggested that cordycepin, an analog of adenosine, is virtually taken up by the thiamin transport system of growing yeast cells; thus the drug sensitivity is decreased by the presence of thiamin in the growth medium.