Hereditarily asplenic mice and normal litternates were transplanted subcatneously with 20x10(6) congenic spleen cells at birth. Eighty per cent of the recipients developed subcutaneous nodules closely resembling the spleen structure seen ia a normal mouse. The growth of the graft was associated with hyperplasis of the majority of the bone marrow cellseries and an increased number of stem cells (CFU-spleen) and agar colony forming cells (CFC) per femoral shaft. Particularly noticeable was the hyperplasia of the granulocytic series which paralleled the increase of CFC number and of serum colony stimulating factor. The presence of the graft did not modify the erthrocytic seroes and the leukocytics of asplenic mice but reduced the average platelet count. It is xonxluded that the growth of the transplant was associated with the hematopoietic alteration described. These results would also indicate humoral or cellular influences of the spleen on the release of or production of marrow cells.