The effects of motorcycle exhaust particulate (MEP) on human cytochrome P-450 (P-450)-dependent monooxygenases were determined using human hepatoma cell line HepG2 and lung carcinoma cell line NCI-H322 treated with organic extracts of MEP from a two-stroke engine. Gas chromatography and mass spectrometry analysis of MEP extract revealed the presence of carcinogens benzo[a]pyrene, benz[a]anthracene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[g,h,i]perylene, chrysene, and indeno[1,2,3-c,d]pyrene in the chemical mixture. Treatment with MEP extract produced concentration- and time-dependent increases of monooxygenase activity in HepG2 cells. Treatment of the cells with 100 µg/ml MEP extract for 24 h markedly increased benzo[a]pyrene hydroxylation, 7-ethoxycoumarin, and 7-ethoxyresorufin O-deethylation activities in microsomes. Immunoblot analysis of microsomal proteins using mouse monoclonal antibody 1-12-3 against P-450 1A1 revealed that MEP extract induced a P-450-immunorelated protein in the hepatoma cells. RNA blot analysis of cellular total RNA using a human P-450 1A1 3?-end cDNA probe showed that MEP extract increased the level of a hybridizable P-450 mRNA. These P-450 1A1 inductive effects of MEP extract were similar to those from treatment with 10 µM benzo[a]pyrene or 3-methylcholanthrene (3-MC) in HepG2 cells. Treatment of lung carcinoma NCI-H322 cells with 100 µg/ml MEP extract, 10 µM benzo[a]pyrene, or 3-MC resulted in induction of monooxygenase activity, protein, and mRNA of P-450 1A1, similar to the induction observed with the hepatoma cells. The present study demonstrates that MEP extract has the ability to induce human hepatic and pulmonary P-450 1A1 in the liver- and lung-derived cell lines, and the induction involves a pretranslational mechanism. Induction of the human hepatic and pulmonary P-450 1A1 in vitro may provide important information in the assessment of MEP metabolism and toxicity in humans.