Taxol is a new antimicrotubule agent that, besides a well known efficacy against solid tumors, has shown activity in non-Hodgkin's lymphomas too. We therefore investigated its in vitro cytotoxic activity against cells from patients affected by chronic lymphocytic leukemia (CLL). Peripheral blood lymphocytes from 46 CLL patients were incubated for four days with Taxol at doses ranging from 0.01 to 10 mM and the cytotoxicity was evaluated by a colorimetric method (XTT). In most samples Taxol was inactive and the IC50 was > 10 mM in 40 out of 46 patients. It is worthwhile noting that four of the six in vitro responsive patients had unfavourable clinical features. In three unresponsive patients we also observed that Taxol was not able to induce apoptosis in vitro. In conclusion, based on in vitro data, it seems that Taxol is not an active drug in standard CLL but we cannot exclude some efficacy in other more rapidly proliferating lymphoproliferative disorders.