Synchronous CHO cells were obtained by mitotic selection; synchrony was maintained up to the 5th cell cycle. The mitotic cells were seeded into T-25 flasks or P-60 plastic petri dishes, and cultured for 1 h at 37 degrees C, then the cells were treated by X-ray, UV light, and mitomycin C. The cells were then cultured for 2 cell cycles with TPA and BrdUrd and sister-chromatid exchanges (SCE) analyzed by the FPG method. Following X-irradiation, the frequency of induced SCE increased linearly with dose reaching a maximum of 19.8 times the control frequency after 200 rad. With higher doses, the SCE frequency declined. In the presence of TPA, SCE frequencies were 1.8 times control levels for all X-ray doses studied (0-800 rad), the frequency seen in non-irradiated cultures treated with TPA. The induced SCE frequency also increased linearly following treatment with UVL and mitomycin C, reaching levels higher than 1.8 times controls with doses exceeding 2.5 J/m2 UVL or mitomycin C (30 min). In the presence of TPA, the SCE frequencies increased to 1.8 times controls following low UVL and mitomycin C doses, but were not influenced by TPA in the higher dose range (above 2.5 J/m2 or 10(-10) M mitomycin C. Most of the SCE were induced by X-rays during the first S phase after treatment. Following higher UVL doses (5 J/m2), however, the SCE frequency remained elevated (1.5 times controls) for 4 cell cycles after exposure.