A temperature-sensitive (ts) mutant, designated tsFT210, was isolated from a mouse mammary carcinoma cell line, FM3A. The tsFT210 cells grew normally at 33 degrees C (permissive temperature), but more than 80% of the cells were arrested at the G2 phase at 39 degrees C (non-permissive temperature) as revealed by flow-microfluorimetric analysis. DNA replication and synthesis of other macromolecules by this mutant seemed to be normal at 39 degrees C for at least 10 h. However, in this mutant, hyperphosphorylation of H1 histone from the G2 to M phase, which occurs in the normal cell cycle, could not be detected at the non-permissive temperature. This suggests that a gene product which is temperature-sensitive in tsFT210 cells is necessary for hyperphosphorylation of H1 histone and that this gene product may be related to chromosome condensation.