The effect of WIN55212-2, a cannabinoid receptor agonist, on acute inflammation of mouse ear was investigated. We found that topical application of WIN55212-2 suppressed ear swelling induced by 12-O-tetradecanoylphorbol 13-acetate or 2-arachidonoylglycerol. Similar inhibition was observed with CP55940, another cannabinoid receptor agonist, and HU-308, a cannabinoid CB(2) receptor-selective agonist. WIN55212-2 also suppressed the infiltration of leukocytes induced by 12-O-tetradecanoylphorbol 13-acetate. On the other hand, WIN55212-3, an inactive enantiomer of WIN55212-2, exerted only small effects on inflammation. Notably, SR144528, a cannabinoid CB(2) receptor antagonist, also suppressed inflammatory reactions in mouse ear. Thus, both the cannabinoid CB(2) receptor agonist and antagonist are capable of reducing inflammatory reactions. We then investigated the mechanism underlying WIN55212-2-induced suppression of inflammation using cultured cells. We found that the addition of WIN55212-2 together with 2-arachidonoylglycerol blocked 2-arachidonoylglycerol-induced migration of human promyelocytic leukemia HL-60 cells that had been differentiated into macrophage-like cells. The restoration of 2-arachidonoylglycerol-desensitized cells and WIN55212-2-desensitized cells from an anergic condition was examined next. We found that 2-arachidonoylglycerol-treated cells rapidly recovered the capacity to respond to 2-arachidonoylglycerol. On the other hand, the anergic condition toward 2-arachidonoylglycerol continued for a longer period after pretreatment with WIN55212-2. These results suggest that the anti-inflammatory activity of WIN55212-2 is attributable, at least in part, to interference with the actions of the endogenous ligand, 2-arachidonoylglycerol.