Human peripheral blood mononuclear cells from normal healthy volunteers were exposed to elevated temperatures of 41-43 degrees for up to 6 hr. Thereafter, the cells were stimulated with phytohemagglutinin in vitro in order to measure indirectly the surviving fraction. DNA replication in heated cells in response to phytohemagglutinin was found to be a sensitive indicator of thermal injury. Exposure to even 40 degrees for 2 hr lowered thymidine incorporation at early time points after phytohemagglutinin stimulation, but the cells were able to recover from thermal injury after exposure for up to 4 hr at 42 degrees. At 43 degrees, exposure for even 1 to 2 hr caused irreversible damage. The changes in thymidine incorporation were not due to changes in endogenous nucleotide pools since parallel changes were observed in DNA polymerase activity. Thus, the heat sensitivity of normal human lymphocytes could be a limiting factor for use of hyperthermia as an adjunct to radiotherapy and chemotherapy of human cancer.