The ability of thymidine to enhance the frequency of HGPRT-, 6-thioguanine resistant V79 cell mutants has been investigated as a function of post treatment growth interval. We have also studied the relative sensitivities towards exogenous thymidine of wild type and mutant cell lines. Our data imply that increases in mutant frequency following exposure of V79 cells to thymidine can be explained on the basis of a greater sensitivity of wild type cells compared with HGPRT deficient cells.