Four beagles received a standardized dose of 33000-44000 CFU-C per kg b.w. isolated from the peripheral blood of DLA-identical donors. Three had permanent reconstitution of hemopoiesis while in one regeneration was abortive. This indicates that the CFU-C assay is able to predict for the hemopoietic potential of a graft. However, considerable differences in the regeneration pattern of CFU-C in the bone marrow demonstrated that the number of repopulating stem cells was variable.