Both alpha- and beta-endorphin were shown to incorporate radioactive polyamines during incubation in the presence of purified transglutaminase and Ca2+, spermine acting as the best acyl acceptor substrate. The endorphin labelling is dependent on the time of exposure to the enzyme and on the substrate concentration. In the absence of acyl acceptor polyamines, isotopically labelled beta-endorphin gives rise to high molecular weight radioactive polymer(s). Moreover, when different proteins acting as transglutaminase substrates are added, beta-endorphin produces heterologous structures. These data indicate that the glutamine-71 of the beta-lipotropin chain, contained in both alpha- and beta-endorphin, functions as acyl donor substrate for the enzyme and that at least one beta-endorphin lysyl residue can serve as acyl acceptor.