mRNA encoding the kappa opioid receptor gene sequence was identified and isolated from various human lymphocytic cells: CEM x174 (a hybrid of T and B origin) cells, Jurkat-T4 cells, human peripheral blood mononuclear cells (PBMC) and purified CD4+ cells. Analyzing the cDNA sequences of RNA transcripts spanning the putative second extracellular loop, which has reported dynorphin specificity, and the seventh transmembrane domain revealed a 100% and 95% homology in amino acid sequence to corresponding kappa opioid receptor sequences in human placenta and rat brain, respectively. Expression of a similar kappa opioid receptor sequence could be detected in normal monkey PBMC but not in monkey PBMC in which the CD4+/CD8+ cell ratio (or CD4+ cell number) was significantly reduced due to prolonged SIV infection. These findings suggest that human and monkey lymphocytes constitutively express kappa opioid receptor mRNA.