Arachidonic acid metabolism by 5-lipoxygenase leads to production of the potent inflammatory mediators, leukotriene (LT) B 4 and the cysteinyl LT. Relative synthesis of these subclasses of LT, each with different proinflammatory properties, depends on the expression and subsequent activity of LTA 4 hydrolase and LTC 4 synthase, respectively. LTA 4 hydrolase differs from other proteins required for LT synthesis because it is expressed ubiquitously. Also, in vitro studies indicate that it possesses an aminopeptidase activity. Introduction of cysteinyl LT and LTB 4 into animals has shown LTB 4 is a potent chemoattractant, while the cysteinyl LT alter vascular permeability and smooth muscle tone. It has been impossible to determine the relative contributions of these two classes of LT to inflammatory responses in vivo or to define possible synergy resulting from the synthesis of both classes of mediators. To address this question, we have generated LTA 4 hydrolase-deficient mice. These mice develop normally and are healthy. Using these animals, we show that LTA 4 hydrolase is required for the production of LTB 4 in an in vivo inflammatory response. We show that LTB 4 is responsible for the characteristic influx of neutrophils accompanying topical arachidonic acid and that it contributes to the vascular changes seen in this model. In contrast, LTB 4 influences only the cellular component of zymosan A-induced peritonitis. Furthermore, LTA 4 hydrolase-deficient mice are resistant to platelet-activating factor, identifying LTB 4 as one mediator of the physiological changes seen in systemic shock. We do not identify an in vivo role for the aminopeptidase activity of LTA 4 hydrolase.