Tissue distribution of hematopoietic progenitors after cyclophosphamide administration. Bone marrow (BM), spleen, and peripheral blood were obtained 8 days after cyclophosphamide and assayed for colony-forming cell (CFU-C) content. The baseline level of CFU-C present in each tissue is indicated by arrowheads. Four to six age- and sex-matched mice were analyzed. Data represent the mean ± SEM. (▪), Wild type; (▨), G-CSFR (−/−).