A monolayer culture of a clonal cell strain of rat glioma was exposed to 2,450 MHz microwave (continuous wave) radiation. The clonal cell strain was the C6 strain established by Benda, et al. (22). The temperature of the medium was measured with a thermistor probe placed under each culture bottle, and it was maintained at 37℃ or at 38.5℃ during 10-hour irradiation, and similarly at 40℃ during 5-hour irradiation. After each irradiation, the culture bottle was incubated at 37℃ for 48 hours. The cell growth of C6 cells exposed to microwave radiation for 10 hours did not show any changes as long as the temperature of the medium was kept lower than 38.5℃. However, when the temperature was kept at 40℃ for 5 hours by microwave irradiation, C6 cells significantly decreased (63%) in numbers on the 2nd day after the treatment (Table 1). Ten days later, these cells resumed their exponential growth and their population doubling time was same as that of nontreated C6 cells (Fig. 9). On the other hand, the treatment of the cells at 40℃ for 5 hours in a conventional incubator resulted in only a 20% decrease in cell numbers and 10-hour treatment at 41.5℃ resulted in a 48 % decrease in the number of C6 cells (Fig. 9). The difference between the effects on cell numbers of microwave radiation and conventional heat incubation suggest that the temperature distribution within the culture bottle might not be uniform under microwave irradiation. Therefore growth inhibition of the cells after hyperthermic (40℃) microwave irradiation was considered probably due to the thermal effects rather than to the nonthermal effects of electromagnetic irradiation. After microwave irradiation, May-Giemsa staining of surviving cells did not reveal any morphological abnormalities (Fig. 6). Feulgen reaction of the cells did not reveal any changes in the number of nuclear DNA particles, either (Fig. 7). In addition, chromosome analysis, performed on the 2nd and the 10th day after microwave radiation, did not show any appreciable chromosome damage. One hundred metaphases of C6 cells were examined for each analysis and 84% of them had the stemline number of 40 chromosomes (Table 2). The karyotype of these cells was not different from that of nontreated C6 cells. In the stemline karyotype, five chromosomes were missing: one chromosome of the A2 pair, one chromosome of the A3 pair, one chromosome of the C12 pair, and two chromosomes from group B. There were also constant gains of three markers (Fig. 8). The present results indicate that 2,450 MHz continuous wave radiation on the C6 cells did not reveal any nonthermal effects of electromagnetic radiation, in contrast to several other reports: Webb and Dodds (47), Yao, et al. (48), Okai (50), Webb and Booth (59), and Heller (67).