The presence of high concentrations of membrane-bound carboxypeptidase M in human, baboon, dog, and rat lung was established by employing a variety of techniques. The activity of the enzyme in the membrane-enriched fractions of human, baboon, dog, and rat lung, measured with fluorescent dansyl substrate (DNS-Ala-Arg), was 198, 261, 484, and 153 nmol/h/mg protein, respectively. This activity in the lung was much higher than that found in the heart, liver, or kidney. The enzyme, optimally active around neutral pH, was completely inhibited by 10 microM 2-mercaptomethyl-3-guanidinoethylthiopropanoic acid and was activated by 1 mM CoCl2 to 170%. Antibody to human carboxypeptidase M immunoprecipitated the solubilized carboxypeptidase from human (98%), baboon (81%), and dog (88%) lung membrane fractions. Carboxypeptidase M is attached to lung membranes by a phosphatidylinositol glycan anchor; thus, it is released with bacterial phospholipase C. Membrane fractions from cultured human pulmonary arterial endothelial cells also contained high carboxypeptidase M activity (254 nmol/h/mg protein). A Northern blot of poly(A)+ RNA from various human tissues showed the presence of a high level of carboxypeptidase M mRNA in human lung and placenta. Finally, immunohistochemistry, employing purified antibody to the enzyme, revealed in fluorescent light microscopy that carboxypeptidase M is present in alveolar type I pneumocytes and in macrophages in apparently lower concentration. In contrast, type II alveolar epithelial cells gave negative results. Because carboxypeptidase M cleaves a variety of active peptides (e.g., bradykinin, anaphylatoxins), it may protect the alveolar surface from the effects of these peptides. In addition, carboxypeptidase M could be a marker enzyme for type I cells.