Cannabinoids have immunomodulatory as well as psychoactive effects. Because the central cannabinoid receptor (cannabinoid CB(1) receptor) is highly expressed in many neuronal tissues and the peripheral cannabinoid receptor (cannabinoid CB(2) receptor) is highly expressed in immune cells, it has been suggested that the central nervous system effects of cannabinoids are mediated by cannabinoid CB(1) receptors and that the immune effects are mediated by cannabinoid CB(2) receptors. To test this hypothesis, we have generated the first mouse strain with a targeted mutation in the cannabinoid CB(2) receptor gene. Binding studies using the highly specific synthetic cannabinoid receptor agonist (-)-cis-3-¿2-Hydroxy-4-(1, 1-dimethylheptyl)phenyl-trans-4-(3-hydroxypropyl)cyclohexanol (¿3HCP 55,940) revealed no residual cannabinoid binding sites in the spleen of the cannabinoid CB(2) receptor knockout mice, while binding in the central nervous system was unchanged. Cannabinoid CB(2) receptor knockout mice, which appear healthy, are fertile and care for their offspring. Fluorescence activated cell sorting (FACS) analysis showed no differences in immune cell populations between cannabinoid CB(2) receptor knockout and wildtype mice. We investigated the immunomodulatory effects of cannabinoids in cannabinoid CB(2) receptor deficient mice using a T cell co-stimulation assay. Delta(9)Tetrahydrocannabinol inhibits helper T cell activation through macrophages derived from wild type, but not from knockout mice, thus indicating that this effect is mediated by the cannabinoid CB(2) receptor. In contrast, central nervous system effects of cannabinoids were not altered in these mice. Our results suggest that cannabinoid CB(2) receptor-specific ligands may be clinically useful in the modulation of macrophage immune function while exhibiting no central nervous system activity. Furthermore, we conclude that the cannabinoid CB(2) receptor knockout mouse is a useful animal model in which to study the role of the cannabinoid system in immunoregulation.