Bone marrow granulocyte colony forming cells (CFU-C) were measured in HLA compatible sibling donor-recipient pairs. The regeneration of granulocytes and CFU-C compartments were also studied in order to evaluate haemopoietic recovery. The number of nucleated bone marrow cells in the donation was 23 +/- 4 X 10 cells, which recipients received (3.5 +/- 0.4) X 10(8) nucleated cells/kg and (1.19 +/- 0.32) X 10(5) CFU-C/kg. This produced a bone marrow reconstitution of (67 +/- 26) X 10(5) CFU-C/kg by day 30. There was a significant correlation between CFU-C/kg and (1) granulocyte count on day 30 (P equal less than 0.05) and (2) the first day of reappearance of neutrophils in the blood (P equal less than 0.05). These results indicate that the speed and completeness of granulocyte regeneration can be predicted by measurement of the size of the CFU-C inoculant in the bone marrow graft.