The cytokinesis-block micronucleus (MN) assay was performed in three mouse tumors: two sarcomas (SaL, MCA) and Lewis lung carcinoma (LLC). To determine the optimal culture durations and cytochalasin B (cyt-B) concentrations to yield the highest proportion of binucleate cells (BC) for each tumor, the influence of the cyt-B concentration (1, 2 and 3 micrograms/ml) and culture duration (24-96) were studied. The amount of BC and the MN frequency was investigated for the different radiation doses (0-4 Gy). Dose response curves were constructed using the optimal culture duration and cyt-B concentration for each tumor. This was 24 h of incubation for MCA and 48 h for SaL and LLC and 2 micrograms/ml of cyt-B. The tumors examined differ in the mean number of spontaneously (0 Gy) occurring MN in binucleate cells. These were 0.008, 0.022 and 0.044 for MCA, SaL and LLC, respectively. The MN frequency increased with radiation dose. LLC was found to be the most radiosensitive, while MCA proved to be the least radiosensitive tumor. The average number of MN/BC at 2 Gy of irradiation (after subtraction of the value at 0 Gy) ranged from 0.05 to 0.36. The highest mean value -0.36 was shown in LLC, the middle-0.20 in SaL, and the lowest-0.05 in MCA tumor. After higher doses of irradiation numerous BC with two and more MN were found in LLC tumor, while they were not frequently observed in MCA tumor. We did not observe an increase in the MN frequency with culture duration or proliferation rate of the tumor cells. MCA has the shortest potential doubling time (Tpot) and had the lowest MN frequency from three examined tumors. The MN assay has promise to be a rapid predictive assay of radiosensitivity.