Long-term therapy of AIDS patients with 3'-azido-3'-deoxythymidine (AZT) is limited by hematopoietic toxicity. While the mechanism(s) of this toxicity remain elusive, various strategies are being developed to reduce these toxic effects including combination therapy with non-myelotoxic anti-human immunodeficiency virus (HIV) drugs and/or administration of protective or rescue agents, such as cytokines and growth factors. Using a physiologically relevant human CD34+ bone marrow cell liquid culture system, a crosslinked human recombinant hemoglobin (rHb), currently in Phase II clinical trials, was investigated for effects on hematopoiesis and for its potential in protecting or reversing AZT-induced hematopoietic toxicity. These investigations demonstrated that 0.01, 0.1, or 1 microM human rHb did not affect the proliferation of erythroid or myeloid lineage cells. A concentration of 1 microM rHb partially protected erythroid lineage cells from an inhibition of proliferation induced by 0.1 and 1 microM AZT. Inhibition of proliferation of cells previously exposed to AZT was not reversed at this concentration. These data suggest that human rHb may be of benefit in reducing the toxic effects of AZT in the bone marrow of AIDS patients.