Macrophage activation is associated with a marked increase in intracellular hydrogen ion generation, necessitating compensatory mechanisms for maintaining intracellular pH. The carbonic anhydrases (CAs) are a family of isozymes which play an important role in cellular pH regulation. Alveolar macrophages were found, by immunohistochemsitry of normal rat lung tissue, to be positive for the cytosolic isozyme CA II. Further, induction of pulmonary inflammation in rats by acute radiation exposure was associated with the infiltration of alveolar macrophages stongly positive for CA II. We therefore hypothesized a role for CA II in activated macrophages, and investigated regulation of CA II expression in this cell type. Immunoblot and immunocytochemical analyses demonstrated strong reactivity of cultured alveolar macrophages to CA II. RNA blot analysis confirmed expression of the CA II gene by this cell type. No CA I expression was detected and expression of the membrane associated isozyme CA IV was confined to the non-adherant macrophage population. In order to determined if CA II expression is regulated during activation, we treated macrophages in culture with γ-interferon (1 to 1000 u/cc) and/or endotoxin (0.01 to 10 μg/cc). Exposure of cultured alveolar macrophages toγ-interferon resulted in no change in CA II mRNA or protein. However exposure to endotoxin resulted in a dose-dependent increase in CA II expression. We conclude that CA II expression increases in response to macrophage activation and speculate that this enzyme is participating in the maintainence of intracellular pH.