The flavin-containing monooxygenase has been purified from mouse and rabbit lung microsomes and shown to be distinct from the flavin-containing monooxygenase found in the liver of the same species. The mouse and rabbit lung monooxygenases have a unique ability to N-oxidize the primary aliphatic amine, n-octylamine, commonly included in microsomal incubations to inhibit cytochrome P-450. In the mouse lung, this compound not only serves as a substrate but is also a positive effector of metabolism. The mouse and rabbit lung enzymes have unusual pH optimum, near 9.8, compared to the liver enzymes which have peaks near pH 8.8. Using antibodies raised in goats, Ouchterlony immunodiffusion analysis indicates that the liver and lung proteins are immunochemically dissimilar.