Gene transfer into human cells using murine amphotropic retroviral vectors is the basic technique used in most current gene therapy studies. The identity of the cell surface receptor for the amphotropic envelope remains unknown and thus its importance in gene transfer is poorly understood. We have measured specific retrovirus binding to cells to study amphotropic virus receptor regulation in human CD34+ bone marrow (BM) progenitors and primitive CD34+CD38- human hematopoietic cells. The rat monoclonal antibody 83A25 recognizes an epitope common to the envelope glycoprotein of all classes of Moloney murine leukemia virus. Indirect fluorescent labeling of 83A25 allows flow cytometric analysis of specific virus-cell interactions and is an indirect measure of specific receptors. Using this assay, amphotropic virus binding to fresh CD34+ cells was minimal. However, when CD34+ cells were cultured with or without growth factors for 4 days, specific binding of amphotropic retrovirus was readily shown. Inclusion of interleukin-3 (IL-3), IL-6, and Steel factor in cultures increased the fluorescence associated with amphotropic virus binding by twofold to four-fold (mean fold increase 2.7 +/- 0.84). Virus binding to CD34+CD38- cells was shown only in those cells culture in IL-3, IL-6, and Steel factor. These results suggest that certain cytokines may cause an increase in the number and/or affinity of amphotropic receptors on primitive human hematopoietic cells. Upregulation of viral receptor expression may be one of the mechanisms by which cytokines enhance gene transfer into primitive BM cells.