The effects of 3'-azido-3'-deoxythymidine (AZT) and 9-(1,3-dihydroxy-2-propoxymethyl)guanine on myeloid and erythroid colony-forming cells were studied by clonogenic assays. Both consistently inhibited granulocyte-macrophage CFU (CFU-GM) and erythroid burst-forming units in a dose-dependent fashion. Concentrations of AZT and 9-(1,3-dihydroxy-2-propoxymethyl)guanine required for 50% inhibition of CFU-GM were, respectively, 0.9 +/- 0.1 and 2.7 +/- 0.5 microM; those required for 90% inhibition were, respectively, 34.0 +/- 2.8 and 35.7 +/- 3.6 microM. Erythroid burst-forming units were less sensitive to high concentrations of AZT than were CFU-GM.