A temperature sensitive growth mutant, 13B11 derived from CHO-K1 has been partially characterized. Upon shift to the nonpermissive temperature (39 degrees C) DNA synthesis slows down, but it is partly resumed after prolonged incubation at 39 degrees C. The radioautographic results suggest that reduction in the rate of DNA synthesis is mainly due to decrease in the proportion of DNA synthesizing cells and that increase in that rate observed afterwards is due to appearance of cells that enter S. Synchronized mutant cells incubated at 39 degrees C from the beginning of G1 enter S with the delay of 6 hours. By incubating the cells at 39 degrees C for a restricted period during the G1 phase, some processes in late G1 are found to be susceptible to the high temperature. Mutant cells shifted up in the middle of S performed cell division whereas cells up-shifted in G1 phase did not, although about a half of these cells divided when they were shifted down in the presence of hydroxyurea. The analysis of DNA content of the cells cultured at 39 degrees C for more than one generation time, revealed the accumulation of nuclei containing DNA in amount of almost 4C level. Accordingly in this mutant the process necessary for cell division is also temperature sensitive. Considering the frequency of the appearance of spontaneous revertants (2 X 10(16)), the apparent two lesions of this mutant might be ascribed to the single mutation.