Hematopoietic dysfunction with peripheral cytopenias is a common complication of human immunodeficiency virus (HIV) infection. Symptomatic anemia is the most common cytopenia and occurs in the presence and absence of myelosuppressive drug therapy such as zidovudine. Drug-induced neutropenia and immune thrombocytopenia are also frequent and occur in up to 50% of acquired immunodeficiency syndrome (AIDS) patients. Attempts to reduce the impact of bone marrow failure have focused on dose reduction of zidovudine, ganciclovir, and chemotherapy, and the use of recombinant hematopoietic hormones such as erythropoietin (EPO) and granulocyte colony-stimulating factor (G-CSF). Despite these maneuvers, approximately 30% of patients with AIDS receiving zidovudine will become transfusion-dependent. This has led to investigations of other cytokines that may increase blood cell formation. The recent identification of decreased number and proliferation of hematopoietic progenitors in patients with HIV infection suggests that agents which have activity on progenitor cell pools may have clinical utility. We demonstrate that human stem cell factor (HuSCF) increases burst-forming unit-erythroid (BFU-E), colony-forming unit-granulocyte-monocyte (CFU-GM), and CFU-Mix formation in vitro in normal and HIV-infected individuals. HuSCF also decreases the sensitivity of BFU-E to inhibition by zidovudine without altering HIV replication in lymphocytes or monocytes, altering peripheral blood mononuclear cell proliferation to phytohemagglutinin (PHA) and interleukin-2 (IL-2) or altering the effectiveness of zidovudine or dideoxyinosine in inhibiting HIV replication in lymphocytes or monocytes. These studies suggest that HuSCF may have clinical utility in HIV infection as an adjunctive treatment for HIV-related cytopenias.