1. Polyclonal antibodies raised against rabbit liver cytochrome P-450 isozymes form 4 and 6 have been used to probe human liver microsomes for analogous proteins using the Western blot technique. 2. Anti-Form 4 IgG recognized a protein in human liver microsomes from six subjects of identical molecular weight to purified rabbit liver cytochrome P-450 Form 4. 3. The equivalent content of cytochrome P-450 Form 4 in the same microsomes ranged from 1̇1 to 9̇1 pmol per mg protein. 4. Anti-Form 6 IgG recognized a protein in human liver microsomes from the same six subjects of slightly higher molecular weight than purified rabbit cytochrome P-450 Form 6. 5. The equivalent content of cytochrome P-450 Form 6 in the above microsomes ranged from 1̇6 to 3̇8 pmol per mg protein. 6. No significant correlations were observed between equivalent cytochrome P-450 Forms 4 and 6 content and 2-acetylaminofluorene N-hydroxylase, aminopyrine N-demethylase, benzyprene and aniline hydroxylase activities in liver microsomes from the six subjects tested.