Two diploid cell lines of human origin were irradiated in vitro. Repair of sublethal damage was observed in both lines when they were exposed to a single gamma-ray dose and incubated at 37 degrees C prior to receiving a graded second dose. In addition cells were irradiated at a low dose rate (56 or 76 rad/hr) and survival was compared to that after an acute exposure. A dose-rate effect was demonstrated. Sublethal damage repair was also studied using CHO or V-79 Chinese hamster cells, and the data suggest that the degree of repair is similar in human and rodent cell lines at photon doses up to 500 rad. The repair of sublethal damage can be a significant factor in the survival of human cells in vitro for fractionated doses.