Three isozymes of cytochrome P-450 have been isolated from liver microsomes of untreated mice. Two of the isozymes catalyze the conversion of naphthalene to alpha-naphthol. Antisera have been produced in rabbits against both isozymes, but in their impure state each antiserum reacts with both isozymes. Both antisera, however, inhibit the metabolism of naphthalene by mouse lung microsomes. They thus may be useful in helping to determine whether the two isozymes form different stereoisomers of naphthalene-1,2-oxide.