We studied the toxic effects of azidothymidine (AZT) on the hemopoietic colony growth (CFU-E, BFU-E and CFU-GM) of bone marrow in a murine model of AIDS (MAIDS). A sparing effect by recombinant hemoglobin (r-Hb) on AZT suppression of MAIDS bone marrow was found when 10 µM of r-Hb was included in bone marrow cultures. The AZT toxicity dose response curve showed that at a concentration of 0.1 µM, AZT inhibited CFU-E by 66%, BFU-E by 55% and CFU-GM by 67%. The addition of r-Hb (10 µM) to AZT-treated cultures stimulated CFU-E, BFU-E and CFU-GM by 89, 125 and 160%, respectively, as compared with AZT-treated (control). The addition of r-Hb to non AZT-treated cultures showed further stimulation of CFU-E, BFU-E and CFU-GM to 100, 160 and 187% of the control, respectively. These results indicate that exogenous r-Hb reverses AZT-induced hemopoietic toxicity and may prove to be useful in ameliorating AZT toxicity in immunosuppressive diseases.