Leukotriene B 4 mediates diverse inflammatory diseases through the G protein-coupled receptors BLT1 and BLT2. In this study, we developed mice deficient in BLT1 and BLT2 by simultaneous targeted disruption of these genes. The BLT1/BLT2 double-deficient mice developed normally and peritoneal exudate cells showed no detectable responses to leukotriene B 4 confirming the deletion of the BLT1/BLT2 locus. In a model of collagen-induced arthritis on the C57BL/6 background, the BLT1/BLT2−/− as well as the previously described BLT1−/− animals showed complete protection from disease development. The disease severity correlated well with histopathology, including loss of joint architecture, inflammatory cell infiltration, fibrosis, pannus formation, and bone erosion in joints of BLT1/BLT2+/+ animals and a total absence of disease pathology in leukotriene receptor-deficient mice. Despite these differences, all immunized BLT1−/− and BLT1/BLT2−/− animals had similar serum levels of anti-collagen Abs relative to BLT1/BLT2+/+ animals. Thus, BLT1 may be a useful target for therapies directed at treating inflammation associated with arthritis.