The neuropeptide beta-endorphin can modulate the response of T and B cells to mitogenic or antigenic stimulation. In the present report we describe a novel mechanism by which beta-endorphin can interfere with T cell activation. It is shown here that beta-endorphin can modulate the phorbol ester-induced phosphorylation of the gamma chain of the CD3 complex. The effect of beta-endorphin is dose dependent and appears to be mediated via interaction of beta-endorphin with an opiate receptor on lymphocytes. Evidence is presented that the modulatory effect of beta-endorphin is specific for the phosphorylation of the CD3 gamma chain. beta-Endorphin does not affect the phosphorylation of total cell protein, nor does it have any effect on the phosphorylation of the CD4 determinant on T cells. The possible consequence of a change in CD3 gamma chain phosphorylation is discussed.