In the presence of glutathione and glutathione transferases, microsomal fractions prepared from fresh samples of human lung tissue obtained at resection metabolized naphthalene to naphthalene dihydrodiol and 3 glutathione conjugates at easily measurable rates. Addition of varying amounts of human lung microsomal protein markedly inhibited mouse liver microsome-catalyzed naphthalene metabolism in one sample but not the other. These data show that naphthalene is a good substrate for human pulmonary microsomal monooxygenases. In addition, these studies suggest that there may be an inhibitor, potentially released during tissue homogenization, that makes measurement of human lung xenobiotic metabolism difficult.