Paclitaxel (Taxol) has been shown to be clinically effective in treatment of patients with breast and ovarian cancer. It has also shown promising results in various other solid tumours. Paclitaxel has induced apoptosis in the G2/M phase of the cell cycle in both HL-60 and U937 human leukaemia cells. A recent study has shown a dose-dependent cytotoxicity for both taxanes: paclitaxel (taxol) and docetaxel (Taxotere) on fresh leukaemia cells in primary culture from 16 ALL and four AML patients and proposed their use in treatment of acute leukaemia patients. AML is a heterogeneous disease in which malignant transformation and disease progression occur at the level of CD34 positive cells. Also, the multi-drug resistance gene product, P-glycoprotein is expressed only in CD34 positive AML cells. Therefore, an in vitro evaluation of the efficacy of paclitaxel, a P-glycoprotein substrate, in CD34 positive AML cells is warranted before considering its clinical use in acute leukaemia patients. Since all in vitro studies of paclitaxel reported so far have involved only CD34 negative (HL-60, U937, K562) human AML cells, the aim of the present study was to evaluate paclitaxel efficacy against CD34 positive AML cells. The IC50 of paclitaxel for apoptosis was significantly higher in MHH225 CD34 positive cells (12 +/- 2 microM) than in U937 CD34 negative cells (1.7 +/- 0.2 microM), P < 0.001. Paclitaxel has a significantly weaker cytotoxic effect on CD34 positive AML cells. One log higher concentration of paclitaxel was required in MHH225 CD34 positive AML cells to achieve the same apoptosis level achieved in U937 CD34 negative leukaemia cells. Also, at the high concentration achievable in vivo: 10 microM paclitaxel, only half the MHH225 CD34 positive AML cells were apoptotic versus 72% of U937 CD34 negative leukaemia cells. Clearly, paclitaxel has only weak or modest in vitro efficacy compared with several conventional anti-leukaemia drugs used in AML treatment. The present results support the poor level of in vivo induction of apoptosis achieved during a phase I clinical study with paclitaxel therapy in 26 leukaemia patients. Also, the present results have shown a significant increase in nitric oxide production during paclitaxel-induced apoptosis in U937 monocytic leukaemia cells, confirming the vital role of nitric oxide in mediating paclitaxel-induced apoptosis by monocytic cells. In conclusion, the present study has demonstrated a clear difference between the effect of paclitaxel on CD34 negative and CD34 positive AML cells. Given its poor performance in the phase I clinical study of 26 acute leukaemia patients and the present weak in vitro cytotoxic effect, it is unlikely that paclitaxel will have a role in the treatment of acute leukaemia. Also, the present study emphasises the need to use CD34 positive AML cells such as MHH225 rather than the unsuitable lineage-specific CD34 negative cells such as HL-60 or U937 for in vitro pre-clinical screening of potential novel effective anti-leukaemia agents.