By means of a temperature-sensitive mutant (ts85 strain), we have studied the effect of the decrease in H1 histone phosphorylation on DNA synthesis and chromosome condensation. When ts85 cells were incubated at 39 degrees C (nonpermissive temperature), the rate of H1 histone phosphorylation was decreased gradually and reached half that at 33 degrees C (permissive temperature) by 6-h incubation. Wild-type cells, growth-revertant ts85 cells (ts85R-MN3), and other ts mutants which were arrested mainly at the G2 phase at 39 degrees C had no defects in H1 histone phosphorylation. When ts85 cells were synchronized at the G1/S boundary at 33 degrees C and released from the blockade at 39 degrees C, approximately 70% of cells passed through the S phase and stopped at the G2 phase. The rest were distributed in G1/S to the S phase and mitotic cells were not detected at all. When ts85 cells, synchronized at the G1/S boundary, were further incubated for 8 h at 39 degrees C with a synchronizing agent, the rate of phosphorylation of H1 histone was decreased and the cells were not able to complete DNA synthesis after release from the blockade. Cytofluorometric analysis revealed that the cells had DNA contents of the S phase. Taken together with our earlier data [Matsumoto, Y., Yasuda, H., Mita, S., Marunouchi, T., & Yamada, M. (1980) Nature (London) 184, 181-183], both events, incomplete DNA replication and a defect in chromosome condensation, were thought to be ascribed to the decrease in H1 histone phosphorylation.