The effect of the aging process on the hemopoietic system in senescence-accelerated (SAM-P) mice with respect to the reconstituting ability of hemopoietic cells and the ability of the microenvironment to support hemopoietic reconstitution was investigated by bone marrow transplantation (reconstitution assay). When the bone marrow cells, obtained from young or old mice, were transplanted to lethally irradiated young SAM-P mice no difference in the reconstituting pattern of femoral spleen colony-forming units (CFU-S), splenic CFU-S and splenic granulocyte macrophage colony-forming units (CFU-GM) was observed between the mice transplanted with young and old donor cells. However, the reconstitution of femoral CFU-GM in mice transplanted with old donor cells was delayed compared to that in mice transplanted with young donor cells. Moreover, the recovery of WBC in mice transplanted with old donor cells was noticeably delayed. When the bone marrow cells obtained from young mice were transplanted to young or old recipient mice, no difference in the reconstituting pattern of femoral CFU-S and CFU-GM as well as splenic CFU-S and CPU-GM was observed between young and old recipient mice. However, the recovery of WBC in old recipient mice was noticeably delayed. These data indicate that the functions of both the hemopoietic cells and the hemopoietic microenvironment deteriorated with age in SAM-P mice.