The in vitro effects of paclitaxel (Tx) and docetaxel (Taxotere, Txt) are compared in this study using the adenosine triphosphate cell viability assay (ATP-CVA) in 14 cancer cell lines. Eleven cell lines were sensitive and three were partially sensitive to paclitaxel. Nine cell lines were sensitive, three were partially sensitive and two were resistant to docetaxel. Mean IC50s were 3.7–660 ng/ml paclitaxel and 5.4–540 ng/ml docetaxel. In five sensitive cancer cell lines docetaxel was more active than paclitaxel, and in six sensitive cell lines paclitaxel was more active than docetaxel on a concentration basis. Two cell lines were sensitive to paclitaxel and resistant to docetaxel. In one cell line the two compounds had similar activities. In the ATP-CVA, paclitaxel and docetaxel are very active and are partially non-cross-resistant.