Hydroxyurea (HU), given ip four times, each time at 500 mg/kg in 6-hour intervals, was used to treat DBA/2 mice with Friend murine leukemia virus-induced polycythemia (F-MuLV-P). In these mice a new cell type, found after virus infection, gave rise to erythropoietic colonies in vitro without addition of erythropoietin (Ep) and completely replaced Ep-dependent normal erythropoietic colonies in vitro. The colony-forming units in the spleen (CFUs), the colony-forming units in culture (CFUc), and the erythropoietic colony-forming units (CFUE) were studied. Two days after treatment, CFUs were reduced to about 20% in controls and F-MuLV-P-infected animals, and CFUc were reduced to 6-11% in controls and F-MuLV-P-infected animals. CFUE were not detectable. At day 4 after the first HU dose, when CFUs has regenerated to about normal levels, a sharp rise in Ep-dependent CFUE was seen in the marrow; this rise was not present before HU treatment. The subsequent fall at day 7 coincided with a regeneration of CFUE in the spleen, but in the spleen these CFUE were all Ep-independent. Possibly, the normal Ep-dependent CFUE during regeneration in the marrow might have derived from previously resting CFUs that were not killed by HU. The subsequent conversion to Ep independency could have been due to reinfection by F-MuLV-P persisting in the animal.