We previously reported the cDNA cloning of three new forms of P450, CYP4F4, CYP4F5, and CYP4F6, from a rat brain cDNA library. In the present study, we expressed CYP4F4 andCYP4F5 in Escherichia coli using the pCWOri expression vector with a modification of their N-terminal amino acid sequences and the incorporation of a C-terminal [His]₄tag to aid in purification. CYP4F5 recombinant protein was purified to a specific content of 7.7 nmol/mg protein from the membrane fraction of E. coli and showed ω-hydroxylation activity toward leukotriene B₄(LTB₄), a chemical mediator of inflammation. On the other hand, the solubilized membrane fraction of CYP4F4-expressed recombinant protein catalyzed the ω-hydroxylation of prostaglandin A₁, prostaglandin E₁, and 6-trans-LTB₄as well as LTB₄. The effects of the peroxisome proliferator, clofibrate, on mRNA expression of CYP4F4, 4F5, and 4F6 were studied by Northern blot analysis. The expression levels of the mRNA of these CYP4Fs were shown to be reduced by clofibrate in liver.