Radiosensitivity of examined human neoplastic cell lines was assessed with the aid of MTT assay. Differences between radiosensitive and radioresistant human neoplastic cell lines were as follow: a) radiation-induced apoptosis detected by flow cytometry was apparent in the most radiosensitive (i.e. CH-1 ovarian carcinoma cell line), but not in the radioresistant (i.e. SKOV-3 ovarian carcinoma) cell lines, b) radiation-induced G2/M arrest appeared early after irradiation (6 hours) in both the radioresistant SKOV-3 cells and in the radiosensitive CH-1 human ovarian carcinoma cell line, but a different pattern was observed 24 hours after irradiation with 2 Gy dose with G2/M arrest only in radiosensitive cell line. The radiosensitivity and resistance to radiation-induced apoptosis in the radioresistant human breast carcinoma MDA-MB-231 cell line were similar to those observed in SKOV-3 cells. These data suggest that radiation-induced apoptosis and cell cycle alterations can predict radiosensitivity at least in some examined human malignant cells in vitro.