Trypsinization of human T-lymphocytes removes surface receptors which bind to sheep erythrocytes (E). Human dialyzable leukocytes extracts (DLE) and thymosin (Fraction V) have been shown to significantly increase the rate of regeneration of T-lymphocyte E-receptors. Both physical-chemical and immunochemical results reported herein indicate that the enhancing effect of human DLE preparations on the rate of regeneration of T-lymphocyte E-receptors is due at least in part to the presence of thymosin alpha 1-peptide in these preparations. Thymosin alpha 1-peptide purified from thymosin Fraction V and putative thymosin alpha 1 preparations purified from human DLEs were each active not only in increasing the rate of regeneration of T-lymphocyte E-receptors removed by trypsinization but also were active in vitro in markedly increasing the number of E-rosetting cells in two patients with immunodeficiency disease manifested in part as a reduction in the normal percentage of mature T-lymphocytes capable of forming E-rosettes.