There has been a great deal of interest in the preparation and purification of thymosin, a soluble product of calf thymus (1). This material is believed to be a thymic hormone and like other similar thymic factors described (2) is capable of restoring skin allograft rejection and ability to induce graft-vs-host (GVH)5 disease in neonatally thymectomized mice (3, 4). Further thymosin stimulates lymphoid tissue growth (4), and induces the appearance of T cell markers in bone marrow cells (5). There have been many suggestions that thymosin is critical in preventing autoimmune disease in humans. Indeed levels of thymic hormones are reported low in systemic lupus erythematosus (SLE), and are stated to decrease with age (6). New Zealand NZB × NZW F1 (NZB/W) female mice, after 7 months of age, manifest a large number of immunologic abnormalities including decreased ability to reject skin allografts, decline in capacity of lymphoid cells to respond to allogeneic cells or other mitogens, and diminished primary response to sheep red blood cells (SRBC).