Leukotrienes are derived from arachidonic acid and serve as mediators of inflammation and immediate hypersensitivity. Leukotriene B₄ (LTB₄) and leukotriene C₄ (LTC₄) act through G protein–coupled receptors LTB₄ receptor (BLTR) and Cys-LTR, respectively. To investigate the physiological role of BLTR, we produced mice with a targeted disruption of the BLTR gene. Mice deficient for BLTR (BLTR^−/−) developed normally and had no apparent hematopoietic abnormalities. Peritoneal neutrophils from BLTR^−/− mice displayed normal responses to the inflammatory mediators C5a and platelet-activating factor (PAF) but did not respond to LTB₄ for calcium mobilization or chemotaxis. Additionally, LTB₄ elicited peritoneal neutrophil influx in control but not in BLTR^−/− mice. Thus, BLTR is the sole receptor for LTB₄-induced inflammation in mice. Neutrophil influx in a peritonitis model and acute ear inflammation in response to arachidonic acid was significantly reduced in BLTR^−/− mice. In mice, intravenous administration of PAF induces immediate lethal anaphylaxis. Surprisingly, female BLTR^−/− mice displayed selective survival (6 of 9; P = 0.002) relative to male (1 of 11) mice of PAF-induced anaphylaxis. These results demonstrate the role of BLTR in leukotriene-mediated acute inflammation and an unexpected sex-related involvement in PAF-induced anaphylaxis.