The dynamics of mitotic activity and the frequency of pathologic mitoses and their spectrum in 180 mice bearing Ehrlich solid tumors were studied after exposure to X-rays (XR) (5.6 Gy), ultrasound (US) (2W/cm2, for 5 min) or the combined exposure to both agents delivered in a different sequence. The harmful effect of the above-mentioned agents diminished when they were delivered in the following order: US+XR, XR+US, XR, and US. After the combined treatment the spectrum of the pathologic mitoses widened.