A delta opioid receptor complementary DNA (cDNA) was cloned by expression of cDNA library from activated thymocytes in Cos 7 cells. The deduced amino acid sequence of this receptor was similar to that described in the brain. As analyzed by southern blot hybridization, the delta opioid receptor transcripts are constitutively expressed in unactivated thymocytes. In addition, neither kappa nor mu opioid receptor transcripts were detected in thymocytes, suggesting tissue-specific opioid receptor gene expression in the immune system. The studies represent the first report of a full-length opioid receptor in the immune system.