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Two methods involving labelling cells with bromodeoxyuridine (BrdUrd) have been used to study by flow cytometry the effect of hyperthermia (43 degrees C for up to 1 h) on Chinese hamster V79 cells. One method involved the use of an antibody to BrdUrd after pulse-labelling the cells either before or at time intervals after treatment. In the second method, the cells were incubated continuously in BrdUrd after heat treatment, and the components of the cell cycle were then visualized by staining with a combination of a bis-benzimidazole and ethidium bromide. All three methods showed that heating at 43 degrees C stopped DNA synthesis which, at 37 degrees C, subsequently recovered reaching the normal rate 8-12 h later. The cells in S phase at the time of treatment then progressed to G2 where they were further delayed. Cells heated in G1. after the recommencement of synthesis, progressed around the cycle, albeit slower than in unheated cells. The difference between the cells in G1 and S phases at the time of treatment may account for the greater sensitivity of S phase cells to hyperthermia.