Normal human bone marrow contains cells capable of forming colonies of hemopoietic cells in fibrin clots in diffusion chambers implanted intraperitoneally (i.p.) into irradiated mice. The present paper describes the proliferation of such colony-forming units (CFUD) in cultures in vivo. Cells harvested from diffusion chambers after 1–14 days of culture in 450-R irradiated mice contained CFUD, which formed neutrophilic, eosinophilic, or megakaryocytic colonies when tested by secondary culture in fibrin clot chambers. When bone marrow was precultured in irradiated mice at a concentration of 10(6) cells per chamber, an initial fall in the number of neutrophilic CFUD was observed. This decrease was followed by an increase to a maximum at day 2, and then a secondary decrease. The number of neutrophilic CFUD recovered after 2 days of preculture in irradiated mice varied between 60% and 250% of the number present before preculture. Preculture in nonirradiated mice resulted in a significantly lower recovery of neutrophilic CFUD. In vitro treatment of bone marrow cells with hydroxyurea (OHU) after 2 days of preculture in irradiated mice resulted in a 68% +/- 5% reduction in the number of neutrophilic CFUD. In contrast, OHU had no similar effect on precultures from nonirradiated mice. Both the recovery and sensitivity to OHU of eosinophilic CFUD were independent of host irradiation. Similarly, no effect of host irradiation on the recovery or the 3H-thymidine (3HTdR) labeling index of morphologically recognizable granulocytic cells was observed at day 2. The data suggest an effect of humoral host factor(s) on the proliferation of early precursor cells, which are or become committed to differentiate into the neutrophilic pathway in diffusion chambers.