Normal human non-separated bone marrow cells were cultured in fibrin clots in diffusion chambers implanted intraperitoneally in mice, and harvested at different intervals by a previously described chamber centrifugation technique. This method demonstrates the presence of cell aggregates in the diffusion chambers. When the chambers are implanted in irradiated mice (450 R) and retransplantated into newly irradiated mice every seventh day, a continous increase in number of cells per granulocytopoietic aggregate is observed from day 8 to day 21. This is compatible with the view that the aggregate is observed from day 8 to day 21. This is compatible with the view that the aggregates are colonies. The term 'colony forming unit diffusion chamber' (CFUD) is suggested to denote the ancestor(s) of the colonies. However, formal proof that one colony is derived from one cell is lacking. Preirradiation of mice with 450 R significantly increases the number of neutrophilic granulocytopoietic colonies at day 14, provided the chambers are retransplantated to newly irradiated mice at day 7, indicating that the neutrophilic colony forming unit or its progeny is involved as at least one of the targets of the stimulating effect of host irradiation. In contrast, no effect of host irradiation on the numbers of eosinophilic colonies was observed.