During the purification of rabbit liver microsomal cytochrome P-450 (P-450LM), evidence was obtained for the occurrence of at least four distinct forms. These were distinguished by polyacrylamide gel electrophoresis after treatment with sodium dodecyl sulfate in the presence or absence of mercaptoethanol and were shown to have characteristic spectra as the reduced carbon monoxide complexes. They are designated by their relative electrophoretic mobilities. P-450LM2, which was purified to apparent homogeneity, is induced by phenobarbital and has a subunit molecular weight of 50,000. P-450LM4, which was also extensively purified, is induced by beta-naphthoflavone and has a molecular weight of 54,000. P-450LM1,7, which is induced neither by phenobarbital nor beta-naphthoflavone, is a mixtureMIXTURE OF ABOUT EQUAL AMOUNTS OF TWO FORMS WITH MOLECULAR WEIGHTS OF 47,000 AND 60,000 RESPECTIVELY. Some preparations were obtained containing primarily P-450LM1 or P-450LM7. Benzphetamine, ethylmorphine, and p-nitroanisole are hydroxylated preferentially by P-450LM2, and benzpyrene by P-450LM1,7. Biphenyl is hydroxylated in both positions 2 and 4 by all of the preparations, but the latter position is strongly favored by the action of P-450LM2. Testosterone is hydroxylated primarily in position 16alpha by P-450LM2 and in position 6beta by P-450LM1,7. Although the occurrence of additional forms of the cytochrome with highly similar electrophoretic behavior is not ruled out, it appears that the presence of these forms differing in subunit molecular weight may account for the variety of catalytic activities attributed to this pigment of liver microsomes.