The purpose of the present study was to investigate further the mechanisms underlying subadditive and superadditive interactions of radiation and paclitaxel treatments in vitro. The protocol studied was a fixed radiation treatment (7 Gy) followed either immediately or after a 10 h delay by paclitaxel treatment. Paclitaxel treatment was exposure to either a fixed dose of paclitaxel (10 nM) for varying time intervals up to 25 h, or varying doses of paclitaxel up to 20 nM for a fixed exposure time of 24 h. The cells used were HeLa. Cell survival was assessed by colony forming ability and apoptosis was measured by flow cytometry. The results show that when the paclitaxel exposure time was 24 h superadditive interactions were observed at all paclitaxel doses. A reasonable correlation between surviving fraction as measured by colony forming ability and apoptosis in the attached cells at the end of paclitaxel treatment was observed. The nature of the interaction of radiation and subsequent paclitaxel treatment is critically dependent on the duration of the paclitaxel treatment. (C) 2000 Elsevier Science Ltd. All rights reserved.