We previously reported the cDNA cloning of three new forms of P450,CYP4F4, CYP4F5,andCYP4F6,from a rat brain cDNA library. In the present study, we expressedCYP4F4andCYP4F5inEscherichia coliusing 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.CYP4F5recombinant protein was purified to a specific content of 7.7 nmol/mg protein from the membrane fraction ofE. coliand showed ω-hydroxylation activity toward leukotriene B₄(LTB₄), a chemical mediator of inflammation. On the other hand, the solubilized membrane fraction ofCYP4F4-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 ofCYP4F4, 4F5,and4F6were studied by Northern blot analysis. The expression levels of the mRNA of theseCYP4Fs were shown to be reduced by clofibrate in liver.