New effective cytotoxic agents and combinations are urgently needed in cancer treatment. The enzyme inosine monophosphate dehydrogenase is a potentially useful target for drug development, since its activity has been shown to be amplified in malignant cells. Thiophenfurin, an inhibitor of the enzyme synthesized by us, is endowed with a significant apoptotic activity in promyelocytic leukaemia HL60 cells. Since retinoids were successfully employed in the treatment of patients with leukaemia, demonstrating significant differentiation-inducing and apoptotic effects, we carried out this study to evaluate the effects of the combination of thiophenfurin and several retinoid molecules, acting in different phases of the cell cycle in vitro. The results show that thiophenfurin is capable of eliciting significant S phase-specific antiproliferative effects in different sensitive and resistant cell lines with the IC50s ranging from 6.7 to 26 microM. When HL60 cells were treated with thiophenfurin in combination with retinoids, the effects on cell growth were additive or synergistic, depending on the kind of retinoid used and the sequence of treatment. In particular, we observed additive effects when the cells were exposed to thiophenfurin and all-transretinoic acid either simultaneously or sequentially. Instead, when the new heterocyclic retinoid isoxazole benzoic acid was used, synergism was obtained in the cells treated sequentially. The combination of thiophenfurin and isoxazole benzoic acid determined synergistic apoptotic effects through a mitochondrion-dependent mechanism, suggesting the possible usefulness of this combination in the treatment of leukaemia.