We recently developed a new progenitor assay using murine fetal liver cells that provides a source of pluripotent progenitors, bipotent progenitors, and committed macrophage, megakaryocyte, erythroid, and mast cell progenitors. This clonal cell culture system was used to examine the direct effects of Harvey sarcoma virus on murine hemopoietic progenitors. Very large erythroid colonies containing 100,000 to 200,000 cells were seen in the infected group. Only small erythroid colonies were seen in the uninfected control cultures. The cells in the large erythroid colonies from infected cultures expressed the ras gene as demonstrated by immunofluorescence with a monoclonal antibody to p21, the ras gene product. The infected cells were not immortal since they did not yield secondary colonies upon replating. Sequential observation of individual colonies showed that maturation was not blocked by infection with the virus. The size of other colony types, including granulocyte/macrophage, mast cell, and mixed, was unaffected even though some of these colonies expressed the ras gene. Thus, infection with Harvey sarcoma virus appears to give a growth advantage primarily to committed erythroid progenitors.