Marrow or spleen grafts from aged donor mice produced antibody-forming cells as effectively as did grafts from younger controls in recipients tested 3 to 10 months after the transplantation. All recipients were lethally irradiated, and the T6 chromosome marker was used to demonstrate that they were populated by donor cell lines. Recipients of aged or younger control grafts gave similar responses when stimulated with varying doses of antigen and when tested at different times after the transplantation except in two cases: 1) Recipients of aged spleen grafts gave significantly lower responses than younger controls for the first few weeks after the transplantation. 2) If recipients had been thymectomized before lethal irradiation, aged cell lines (pooled marrow and spleen cells) gave only 37% of the responses of younger controls. Given sufficient time and intact young recipients, immunologic stem cell lines from old donors populated recipients with cells having normal immune responses. These results suggest that age-related immunologic defects are not intrinisically timed in the precursor cell lines that populate the immune system.