Hydroxyurea (HU) appears to increase 3'-azido-3'-deoxythymidine (AZT) antiretroviral activity and cytotoxicity by inhibiting thymidilate synthesis. The combination of AZT and HU may therefore be of clinical usefulness. We evaluated the in vitro hemotoxicities of different combinations of AZT and HU in comparison with the hemotoxicities exerted by either of the two drugs alone. Peripheral blood granulocyte macrophage committed progenitors (CFU-GM) of healthy donors were selected as targets of hemotoxicity. Both AZT and HU separately had a dose-dependent inhibitory effect on the in vitro growth of normal circulating CFU-GM. The combination of the two drugs induced a statistically significant synergistic cytotoxicity. In fact, addition of HU induced a remarkable reduction of AZT ID50. Thus, future clinical application of AZT, HU combination should take into account the greater hemosuppressive action of the combination in respect to that observed following administration of either drug alone.