Young and old mice have been lethally irradiated and injected with syngeneic bone marrow cells from young or old donors to investigate whether self reactivity in old mice results from age-related damage of the radioresistant stromal cells and/or of the bone marrow hematopoietic cells. Thymus and spleen cell repopulations and mitotic responses at 3 months after irradiation are lower in old than in young recipients, suggesting age-related accumulation of stromal cell damage in the thymus as well as in other central and peripheral lymphoid tissues. The same efficiency of bone marrow cells from young and old donors to repopulate the thymus and spleen in recipients of equal age rules out the detrimental effects of aging on stem cells as well as T and B cell precursors. The serum concentration of auto-antibody and glomerular lesions at 3 and 9 months after irradiation were more pronounced in old than in young recipients and displayed no difference in recipients of equal age, regardless of the age of the bone marrow cell donors. These findings support the possibility that age-related damage of stromal cells induces disregulation of the immune system leading to autoimmune phenomena.