Abstract Seven compounds isolated from Fructus Schizandrae chinensis, a traditional Chinese tonic, which is also able to decrease liver lesions by hepatoxic chemicals, are named Schizandrin (Sin) A, B and C, Schizandrol (Sol) A and B and Schizandrer (Ser) A and B. They are dibenzo[ a,c]cyclooctene derivatives. Dimethyl-4,4′-dimethoxy-5,6,5′,6′-dimethylenedioxy-biphenyl-2,2′-dicarboxylate (DDB) is an intermediate for synthesizing Sin C. The interactions of these compounds with rat liver microsomes in vitro have been investigated. Sol A and Sol B gave type I difference spectrum, the other six compounds gave ‘reverse type I’ difference spectrum. When Schizandrins or DDB were incubated with NADPH-reduced microsomes, Sin B, Sin C, Sol B, Ser A and Ser B generated dual Soret peaks at 455–460 nm and 425–430 nm, the other three compounds caused a difference spectrum without 455 nm peak. All these compounds more or less inhibit liver microsomal hydroxylation of benzo[ a]pyrene (BP) demethylation of aminopyrine. Sin B, Sol B and DDB decreased mutagenicity of BP in Ames test.