The minimum time it takes a cell to pass completely through the S phase (MIN S) was examined in human diploid fibroblasts using a sequential [14C]thymidine, [3H]thymidine, [14C]thymidine labeling protocol. MIN S appeared to be around 6-8 h for both WI-38 and MRC-5 cells. In addition, MIN S did not increase in senescent cultures. Since damage to either DNA, its polymerases, or both would result in a reduction in the rate of DNA synthesis and a corresponding increase in MIN S, this suggests that in senescent cultures at least a portion of the cells contain DNA that is relatively undamaged and DNA polymerases that exhibit normal replicative kinetics.