Human peripheral white blood cells, freshly isolated from normal individual donors, were exposed to hyperthermia. Heat-generated DNA strand break damage and white blood cell capacity to repair radiation-induced breaks were determined by a fluorometric alkaline unwinding assay. Strand breaks could be readily detected when white blood cells were incubated in a physiological salt solution at temperatures between 41 degrees and 46 degrees C, for times up to 90 min. The time course of strand break induction at 45 degrees C was characterized by a short initial lag, followed by a period of rapid break induction and subsequently a lower rate. Evidence is presented which suggests that the induction of DNA damage involved a "triggering" mechanism; a short treatment at 45 degrees C (10 to 20 min) initiated a cellular event which led to a rapid increase in the number of strand breaks during subsequent incubation of 37 degrees C. Continuous incubation at 45 degrees C produced less DNA damage than an initial period at 45 degrees C followed by incubation at 37 degrees C. This apparent "triggering phenomenon" was not due to a triggering of the respiratory burst in phagocytic cells, since no O2- could be detected; in fact, a 30-min treatment at 45 degrees C largely blocked the capacity of the cells to respond normally to a soluble stimulator of the respiratory burst. Unlike gamma-ray-induced breaks, 45 degrees C hyperthermia-induced breaks did not rejoin during subsequent incubation for up to 1 h at 37 degrees C. Additionally, 45 degrees C hyperthermia treatment progressively inhibited the ability of the cells to repair subsequent gamma-ray-induced breaks (4 Gy). This inhibition occurred during the period in which 45 degrees C heat rapidly induced strand breaks. Hyperthermia (41 degrees C), which did not trigger strand breaks, did not cause detectable inhibition of this repair capacity. There was no indication that hyperthermia sensitized cells to radiation-induced strand breaks.