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Irradiation of human peripheral blood lymphocytes at Go with 3 Gy of 60Co gamma rays induced chromosomal aberrations such as dicentrics and acentric fragments whereas hyperthermic treatment at 37 43°C for 20 min by water bath immersion produced no such effects. When cells were heated at 43°C for 20 min at various time intervals (1-180 min) after exposure to 3 Gy, the frequency of dicentrics was greatly reduced immediately (1 min) after irradiation compared to cells exposed to radiation alone. The number of dicentrics however, increased to that of cells treated with only radiation when the interval between the two treatments was 20 min or more indicating rejoining of radiation induced chromosome breaks within this period. Acentric fragments and total breaks increased significantly when cells were heated immediately after irradiation but decreased thereafter to control values. Post-irradiation heating at 41'C also reduced the frequency of dicentrics. These studies thus indicate that hyperthermia enhances radiation induced chromosome damage by inhibiting post-irradiation rejoining of chromosome breaks
