(−)-WIN 55,212-3 mesylate is an aminoalkylindole derivative which acts as a competitive neutral antagonist of the human cannabinoid CB2 receptor, blocking both the stimulating action of CP 55,940 and the inverse agonism of SR 144528. (−)-WIN 55,212-3 neither antagonizes nor mimics the effects of Δ9-THC on rat cerebellar membranes, which presumably express the CB1 receptor. (−)-WIN 55,212-3 also weakly antagonizes the melatonin MT1 and muscarinic M4 receptors but has no effect on several other G protein-coupled receptors.