The metabolism of polychlorinated dibenzo-p-dioxins by cytochrome P450 BM-3 from Bacillus megaterium and a mutant enzyme of it (AL(4)V; Ala74Gly, Phe87Val, Leu188GIn triple mutant) was examined. Both purified enzymes metabolized 1-monochloro-, 2,3-dichloro-, and 2,3,7-trichloro-dibenzo-p-dioxin, but not 2,3,7,8-tetrachloro-dibenzo-p-dloxin. The mutant AL(4)V had 2-12 times higher activity than the wild-type P450 BM-3 towards polychlorinated dibenzo-p-dioxins. The products were hydroxylated at an unsubstituted position and/or showing migration of the chloride and were less toxic derivatives with lower than 10% toxicity of the original compounds.