The autoradiographic detection of thioguanine-resistant (TGR) cells has been proposed as a method for measurement of in vivo mutation frequency. This proposition was tested by combining the autoradiographic technique with the technique of long-term lymphocyte culture. Irradiation of lymphocytes on day 0 following by culture showed a dose-related increase the frequency of TGR cells with an expression time of approx. 7 days. There was a suggestion that mis-sense mutations as well as non-sense mutations and deletions were increased. Incubation with aminopterin for 10 days prevented the increase in TGR cells after irradiation and probably decreased the frequency of TGR cells prior to irradiation. The results suggest that many or most of the TGR cells measured autoradiographically are mutant cells. However, the variability observed with the method imposes limitations on its use and suggests that it will be of more value in studying populations rather than individuals.