Ecteinascidine-743 (ET-743) and aplidine are two marine-derived antineoplastics currently in phase II development. With the aim of evaluating whether in vitro haematopoietic studies can predict the toxicity of these two drugs in patients, human bone marrow (BM) samples were incubated with these drugs under conditions which mimicked the administration exposures used in the clinics. As it was observed in different cancer cell lines, ET-743 was more toxic on an equimolar basis in human hematopoietic progenitors (inhibitory concentration reducing the viability to 50% after 24 h exposures; IC50(24h): 10-50 nM) compared with doxorubicin (IC50(24h) values: 280-460 nM), used as a control anticancer drug. In contrast to the high haematotoxic effects observed for ET-743, similar IC values were obtained for aplidine (IC50(24h): 150-530 nM) compared with doxorubicin. For both ET-743 and aplidine, the megakaryocytic progenitor was the most sensitive, compared with the other haematopoietic progenitors (IC50 values were 3- to 5-fold lower in the CFU-Megs compared with the CFU-GMs). The observation that the Cmax observed in patients treated with the aplidine maximum tolerated dose (MTD) (7.1 nM) was 21-75 fold lower than the IC50(24h) value observed for the different haematopoietic progenitors is highly consistent with the lack of haematotoxicity observed in patients treated with this drug. In the case of ET-743, differences between the Cmax value corresponding to the MTD (2.6 nM) and the in vitro IC50 values corresponding to the different progenitors were much lower (4-19-fold), also consistent with the haematotoxicity that was observed in patients treated at recommended doses (RDs) and MTDs. Although CFU-Megs were more sensitive than CFU-GM progenitors to ET-743 in vitro, clinical data showed that neutropenic events were more frequent than thrombocytopenic episodes. Aiming to further improve the predictive value of in vitro IC values corresponding to the different haematopoietic progenitors, additional refinement parameters derived from pharmacokinetic and animal studies are proposed.