Brain-type cannabinoid receptor (CB1) mRNA has been demonstrated in several peripheral tissues; however, the function of this message is not clear. In the present study, we examined the levels of CB1 mRNA and receptor protein in stimulated immune cells to link message and protein expression with cell activation. Consistent with previous reports, immune cell lines from human and mouse were positive by reverse transcription-polymerase chain reaction for CB1 mRNA; however, one cell line, Jurkat, was only weakly positive. Mitogen activation of Jurkat cells, however, increased CB1 mRNA within 2 hr after stimulation and equilibrium binding studies, using Jurkat membranes, showed [3H]CP55,940 specific binding was negative early after mitogen stimulation but positive at 40 hr poststimulation. To investigate whether this observed increase in CB1 mRNA and specific binding activity was associated with expression of the CB1 protein, polyclonal antibodies were produced to a fusion protein consisting of glutathione S-transferase and a 342 amino acid portion (residues 33 through 374) of the CB1 protein. Western blotting analysis showed expression of several immunoreactive proteins on membranes from mitogen-activated Jurkat cells, but not on membranes from unstimulated cells. These results demonstrate a link between the level of CB1 mRNA and surface protein in activated immune cells, suggesting the possibility of a functional role of CB1 in immune cell activation.