A radioimmunoassay, developed for thymosin alpha 1, can also be utilized for the quantitation of the intact native polypeptide, prothymosin alpha, which contains the thymosin alpha 1 sequence at its NH2-terminus (Haritos et al., 1984a). The major epitope was characterized and found to include residues 1-10 at the NH2-terminus of thymosin alpha 1. As little as 5 pmol of prothymosin alpha can be detected in tissue extracts with this radioimmunoassay.