Paclitaxel dose responses in culture have been investigated alone and in association with cytosine arabinoside (ARA-C) and all-trans retinoic acid (ATRA), with the objective of identifying a role for paclitaxel in the treatment of acute myeloblastic leukaemia (AML). Initial studies were done to determine if paclitaxel dose responses of AML blast cell precursors were altered by regulatory compounds known to modify the dose responses of ARA-C. In contrast to ARA-C, paclitaxel dose responses were independent of cell culture method, the growth factors G-CSF and GM-CSF, and the ligands all-trans retinoic acid (ATRA) and hydrocortisone. Most blast cell populations were sensitive to paclitaxel; compared with normal marrow progenitors the dose responses were markedly heterogenous with some more, and others less, sensitive. Remission marrow progenitor paclitaxel responses resembled those of AML blasts in heterogeneity. The cell culture model tested the effect of pacliataxel and ATRA on the ARA-C dose responses of OCI/ AML-5; paclitaxel exposure was either before or after ARA-C to test for an effect of schedule; ATRA was added to the MEC cultures after paclitaxel and ARA-C. Repeat experiments were done to test three dose levels each of paclitaxel and ATRA. When paclitaxel was given after ARA-C, synergism was found for all but one of the dose combinations tested; only three examples of synergy were seen when paclitaxel preceded ARA-C. The studies justify trials combining ARA-C, paclitaxel and ATRA using a schedule suggested by the cell culture findings.