Heme and a series of synthetic heme analogs were tested for inhibition of human immunodeficiency virus-1 (HIV-1) reverse transcriptase (RT) activity. Heme and the protoporphyrin complexes of cadmium, magnesium, and tin significantly inhibited HIV-1 RT, whereas other metalloporphyrins had a lesser or no effect on the enzyme. The mechanism of inhibition was examined with respect to heme and tin protoporphyrin (SnPP), as both compounds have been utilized clinically as treatment for noninfectious disorders. Heme and SnPP inhibited HIV-1 RT in a noncompetitive manner with respect to deoxythymidine triphosphate. Inhibition depended in part on the protoporphyrin structure, because the mesoderivatives of the heme analogs essentially were without effect. Heme also markedly enhanced the inhibitory effect of azidothymidine (zidovudine, AZT) on HIV-1 RT, and the combination of the two compounds showed synergy in inhibiting HIV-1 RT. HIV-1 RT was used to reverse transcribe the glyceraldehyde phosphate dehydrogenase (GAPDH) gene from human kidney. Subsequently, GAPDH cDNA was amplified with Taq polymerase, and electrophoresis showed that HIV-1 RT catalyzed the reverse transcription of human mRNA at a rate comparable to that of Moloney murine leukemia virus. Heme and SnPP prevented cDNA synthesis by HIV-1 RT in this RT-polymerase chain reaction assay. We also examined the effects of these compounds on normal human bone marrow function. Heme stimulated both erythroid and myeloid progenitor colony formation, whereas SnPP was essentially without effect. In contrast, ZnPP had a suppressive effect on hematopoiesis. Finally, we show that heme has a sparing effect against the myelotoxicity of AZT. The results of these studies raise the possibility that combination therapy with AZT and heme, or heme plus an inhibitor of heme catabolism, might have therapeutic potential in the acquired immunodeficiency syndrome.