Growth of murine spontaneous and transplanted AKR T-cell lymphomas results in marked elevations of serum immunoreactive thymosin alpha 1. Thymosin alpha 1 is one of the peptide hormones believed to be secreted primarily by the thymic epithelium. This elevation, however, is not mediated by the thymus but rather, seems to be directly associated with the tumor cells. Growth of a B-cell lymphoma does not generate elevated immunoreactive thymosin alpha 1 in the serum, thus, a thymosin alpha 1-like peptide is selectively associated with these T-cell lymphomas. The possible relationship between expression of T-leukemia viruses and alpha 1 expression is discussed.