To investigate the effects of mutant ras expression on the growth and differentiation of normal human bone marrow, we used retrovirus-mediated gene transfer. A retrovirus (HR-1) containing a mutant ras gene (H12-ras) in addition to the selectable neo gene was transferred by cocultivation of a packaging cell line with long-term cultures of normal human bone marrow. Controls were established by cocultivating aliquots of the same bone marrow with a retrovirus (VSN-2) containing only neo. The efficiency of gene transfer, as determined by the percentage of G418-resistant colony-forming units-granulocyte/macrophage (CFU-GM) immediately after termination of cocultivation, was similar: 8 +/- 4% with HR-1 and 5 +/- 3% with VSN-2. After a further week in long-term culture, there was an increase in the number and percentage of G418-resistant CFU-GM in both the HR-1-infected and VSN-2-infected marrows. Thereafter, the numbers of G418-resistant CFU-GM declined, becoming undetectable at 4 weeks. The time course of the production of G418-resistant colonies was not significantly different in HR-1- and VSN-2-infected marrows, indicating that H12-ras did not alter the proliferation of normal CFU-GM. However, the total cellularity of HR-1-infected marrow cultures was significantly greater than that of VSN-2-infected marrow cultures. This was due to increased cellular proliferation of HR-1-infected cultured cells, since differential counts showed a significant increase in myeloid blast cells together with a slight reduction in mature myeloid cells in HR-1-infected marrow compared to baseline and to VSN-2-infected marrow. No leukemic blast cell colonies were grown from HR-1-infected marrows or control marrows, and HR-1 infection did not confer serum independence. These data show successful retroviral infection of normal bone marrow progenitor cells and suggest that expression of mutant H12-ras in such cells results in enhanced proliferation of early myeloid cells at the expense of differentiation.