In vitro, murine interleukin 5 (IL-5) acts as a colony-stimulating factor for eosinophils and induces B cells to proliferate and secrete antibody. In order to assess the biological effects of IL-5 in vivo, we transplanted lethally irradiated mice with bone marrow cells infected with a recombinant retrovirus bearing the IL-5 coding sequence. Within 2 weeks the peripheral blood of recipient mice exhibited a marked eosinophilia which persisted for at least 12 months, and an excess number of eosinophils was also evident in the bone marrow, spleen, liver, lung, and gut. Although no changes could be detected in the conventional B lymphocyte population, the peritoneum was replete with B cells characteristic of the Ly-1 lineage. Despite these expanded cell populations, mice remained healthy for 12 months after transplantation. These results suggest that IL-5 acts primarily on eosinophils and B cells of the Ly-1 lineage and that persistent overproduction of these cell types is not pathogenic.