Five different members of the flavin-dependent monooxygenase gene family from different animal species have been described to date. We report the purification and characterization of two FMOs from sheep liver. The predominant isoform was purified 240-fold and was homogenous in SDS PAGE. Its molecular weight (MW) was 58 KDa. The N-terminal sequence, the cross-reactivity with anti-rat FMO3 antibodies, and the catalytic properties such the ability to metabolize trimethylamine (TMA) and to stereoselectively produce L-methionine-d-sulfoxide from L-methionine, are all consistent with this protein being an FMO3. The minor form has a MW of 59 KDa and cross-reacts with anti-rat FMO1 antibodies. The methimazole S-oxidase activity catalyzed by this form was not inhibited by TMA but was inhibited by imipramine. These facts imply that this protein is an FMO1. The expression profile of FMO in sheep liver is similar to that in the human or the mouse but differs from the rat profile.