Several of parameters seeking to increase the infection frequency of normal haemopoietic cells by recombinant retroviruses containing a drug selectable marker and/or a haemopoietic growth factor gene have been compared. By using bone marrow cells pretreated with 5-fluorouracil, cocultivation of cells for at least 4 days with virus producing fibroblasts, and addition of pokeweed mitogen stimulated spleen cell conditioned medium, from 30 to 100% of haemopoietic spleen colony forming cells (CFU-S) could be infected. Because drug (G-418) selection was found to select subsets of CFU-S, a retrovirus lacking the drug selectable marker Neor, but containing a GM-CSF gene, was used to study the effects of endogenous GM-CSF production on CFU-S differentiation. Infected CFU-S produced equivalent numbers of erythroid and neutrophil-macrophage committed progenitor cells to control uninfected CFU-S. However, the progeny of infected CFU-S produced GM-CSF, some cells grew autonomously and neutrophil numbers were greatly increased at the expense of erythroblasts. These results suggest that unregulated GM-CSF production, although not altering commitment, alters the amplication phase following commitment.