By selecting variants of Chinese hamster cells that were resistant to 6-thioguanine at 39 degrees C, but which would continue to grow in HAT medium at 33 degrees C, we have isolated cell lines with thermosensitive phenotypes. These clones form colonies in HAT medium and incorporate 14-C-hypoxanthine much more efficiently at 33 degrees C than at 39 degrees C. The specific activity of hypoxanthine-guanine phosphoribo-syltransferase is at least 10 times higher in variant cells grown at 33 degrees C than in those grown in 39 degrees C, and the enzymes from the variant clones are inactivated in vitro at 39 degrees C 7-9 times more rapidly than is the enzyme from wild-type cells. The results are consistent with the conclusion that the selected clones have missense mutations in the structural gene for the enzyme.