The existence of sigma receptors on lymphocytes and thymocytes was characterized using [3H](+)-pentazocine. [3H](+)-Pentazocine specifically labels high affinity sigma-type binding sites on T- and B-enriched lymphocyte membranes. The binding is saturable with T lymphocyte sites having a KD value of 401 ± 85 nM and B lymphocyte sites having a KD value of 302 ± 46 nM. Likewise, saturable high (KD1 277 ± 92 nM) and low (KD2 2.5 ± 1.2 μM) affinity sites for [3H](+)-pentazocine are found on thymocytes as well. In competition studies with lymphocytes, the rank order of potency for competing ligands is (+)-pentazocine = N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrolidinyl)ethylamine (BD1008) > 1R,2S-(+)-cis-N-(2-(3,4-dichlorophenyl)ethyl]-2-(1-pyrrolidinyl)cyclohexylamine (LR132 ≥ (-)-pentazocine ≥ phenazocine > (±)-trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolinyl)cyclohexyl]benzeneacetamide methanesulphonate (if(U-50,488H) = phencylidine ≥ haloperidol = 1,3-di-(o)-tolylguanidine. In competition studies with thymocytes, the rank order of potency for competing ligands is (+)-pentazocine = BD1008 ≥ phenazocine > haloperidol > 1,3-di-(o)-tolylguanidine > phencyclidine > (-)-pentazocine. These compounds were also investigated as potential regulatory molecules in mitogen-stimulated lymphocyte proliferation assays. Of the compounds tested, phencyclidine, 1,3-di-(o)-tolylguanidine, haloperidol, and (+)-pentazocine suppress concanavalin A-induced proliferation at high (10-5 M) concentrations while (-)-pentazocine is inactive. When pokeweed mitogen or lipopolysaccharide are used, these compounds enhance or suppress lymphocyte proliferation depending on the mitogen and concentration of ligand. These results indicate a stereoselective receptor for (+)-pentazocine which is coupled to biological processes of lymphocytes. © 1991.