The orphan G protein coupled receptor, GPR55, has gained notoriety because of its putative identification as a cannabinoid receptor subtype. The significance of this assignment should not be underestimated given the importance of cannabinoids to drug abuse and the potential utility of cannabinoid ligands in treating behavioral disorders leading to conditions such as obesity. The validity of assigning GPR55 to the cannabinoid family is problematic based upon the discovery that endogenous compounds not related to cannabinoid receptor ligands also bind and signal through GPR55. As a consequence, it is important to identify GPR55-selective ligands that can precisely define GPR55’s role in regulating addictive behaviors. Cannabidiol has been reported as a GPR55 antagonist in the literature to GPR55 activation by O1062, but this could not be confirmed our laboratory (Dr. Abood & Dr. Barak). There are no other known small molecule antagonists of GPR55 reported to date, only the CB1 inverse agonist/antagonists SR141716A (3.9 μM) and AM251 (9.6 μM). We have identified 3 potent and selective antagonists for GPR55, that represent 3 different chemical core scaffolds: 1) a quinoline aryl sulfonamide, ML193 (CID1261822) with a 221 nM potency for GPR55 and >145-fold, >27-fold and >145-fold antagonist selectivity against GPR35, CB1 and CB2, respectively and >145-fold agonist selectivity against all of these counter-receptors; 2) a thienopyrimidine, ML192 (CID1434953) with 1080 nM potency for GPR55 and >45-fold antagonist and agonist selectivity against GPR35, CB1 and CB2; and 3) a piperadinyloxadiazolone, ML191 (23612552) with 160 nM potency for GPR55 and >100-fold selectivity against GPR35, CB1 and CB2. All three probes also are active in inhibiting the downstream responses of ERK phosphorylation and PKC β II translocation.