The effects of Leukotriene B4 (LTB4) are mediated by two known receptors, BLT1 and BLT2. LTB4 is a high affinity ligand for BLT1, and many of its pro-inflammatory effects are believed to be transduced through this receptor. The BLT2 is more enigmatic, in that LTB4 is not a high-affinity ligand, nor is it clear that BLT2 activation promotes inflammation. LTB4 ethanolamide (LTB4-EA) is a theoretical 5-LO metabolite of arachidonoyl ethanolamide (AEA). In CHO cells transfected with human BLTR1, LTB4-EA was a potent antagonist with about three times greater affinity for the receptor than LTB4 (Ki = 1.22 nM versus 3.88 nM). LTB4-EA antagonizes the LTB4-induced contractions of guinea pig lung parenchyma with an EC50 of 10 nM. LTB4-EA thus represents a potential endogenous anti-inflammatory compound functioning as a natural antagonist of BLTR1.