All rats were exposed to whole-body irradiation (1,000 rads by X-rays) to induce aplastic marrows. Control rats received no further treatment. Experimental rats were conjugated three days after irradiation with a non-irradiated healthy litter mate by aortic anastomosis. Control rats died within seven days due to irradiation injury. In these animals, bone marrow reticulum cells were easily distinguished from other stromal cells, such as macrophages or sinusoidal endothelial cells. Three to seven days after irradiation, reticulum cells accumulated lipid droplets in the cytoplasm and showed increases in size and number, resulting in fatty aplastic marrows. On the other hand, parabiosed rats showed hemopoietic recovery and lower levels of lipid droplets in reticulum cells at five to eight days after parabiosis. Progressive fibrosis followed after irradiation but a regression followed after parabiosis. This suggested that fibroblastoid reticulum cells aided in producing collagen fibers. It was also suggested that bone marrow stromal cells, namely, perisinusoidal and intersinusoidal reticulum cells, fat-storage cells, and fibroblastoid cells, shared a common cellular origin. The deserted alterations in reticulum cells after irradiation and the hemopoietic recovery after parabiosis might be due to hemopoietic cells stimulating reticulum cells to induce and sustain the hemopoiesis.