Five lignans, neojusticin A (neo A), neojusticin B (neo B), justicidin A (jus A), justicidin B (jus B), and chinensinaphthol methyl ether (CME) were isolated from the ethanol extract of Justicia procumbens (J.p.), which has been used as a herbal remedy in traditional Chinese medicine. The in vitro effects of lignans on rat hepatic cytochrome P450-catalyzed oxidations were studied. Addition of 10 mM lignans caused 21% to 57% decreases of microsomal benzo(a)pyrene hydroxylation (AHH) activity. Among these lignans, neo B had the strongest inhibitory effect on AHH activity with an IC₅₀ of 6±1 mM. Lignans at 10 mM decreased 7-ethoxyresorufin O-deethylation activities by 20% to 48%. Neo B at 10 mM caused a 70% decrease of 7-methoxyresorufin O-demethylation activity whereas this oxidation activity was relatively less affected by other lignans. Lignans (10 mM) also decreased testosterone 6b-hydroxylation activity by 19% to 74% and neo B had the least inhibitory effect on this activity. Kinetic analysis of AHH activity revealed that neo B was a mixed type inhibitor of competitive and noncompetitive characteristics with K_i and K_I of 2 mM and 11 mM, respectively. With NADPH as the variable cofactor, neo B showed a uncompetitive type of inhibition of AHH activity. These in vitro results suggested that lignans from J. p. inhibited monooxygenase activities differentially and neo B might have a role in diminishing the oxidative activation of benzo(a)pyrene.