Previous studies have shown that morphine stimulates simian immunodeficiency virus (SIV) replication in SIV-infected human CE x174 cells through a mechanism of delaying the lysis of infected cells (biochem. Biophys. Res. Commun. 195:1165-1173, 1993). Using SIV infection of rhesus monkeys as an animal model to study HIV and AIDS, we have also found that morphine treatment of SIVMAC239-infected animals has resulted in an increased rate of virus replication by delaying t cell apoptosis and an increased rate of viral mutation (cell. Mol. Biol. Res. , In press, 1995). The present study describes the identification of brain-like opioid receptor sequences in rna transcripts of both gem x174 cells and monkey lymphocytes. Study of the gene sequence of a lymphocyte opioid receptor encompassing the third transmembrane domain and the third cytoplasmic loop indicates a 96% homology in amino acid composition to the delta opioid receptor in brain cells. Expression of such an opioid receptor sequence in lymphocytic cells is constitutive, since it could be detected in both saline-treated and morphine-treated monkeys as well as in morphine-treated monkeys after toxification.