Using a partially purified preparation, thymosin fraction 5, we have documented that thymosin can correct many of the immunological deficiencies resulting from the lack of thymosin function in animal models and in humans. Ongoing studies indicate that there is a family of biologically active peptides within fraction 5 that act on T-cell subpopulations to maintain normal immunological reactivity. Several of these peptides have been purified to homogeneity. Two peptides, thymosin alpha 1 and beta 4, have been sequenced and chemically synthesized. Thymosin fraction 5 has been used in most clinical trials reported to date, including children with immunodeficiency disease and patients with autoimmune diseases and cancer. Most recently, the National Cancer Institute has initiated a number of Phase I and Phase II clinical trials with thymosin fraction 5 and synthetic alpha 1 as part of a new Biological Response Modifier Program. Preliminary results from two of these studies look encouraging.