The present report describes several characteristics of rabbit microsomal cytochrome P-450 form 3b isolated from liver microsomes prepared from an incipient inbred strain, IIIVO/J. Whereas most preparations of P-450 3b isolated from outbred New Zealand White (NZW) rabbits catalyze both 6 beta- and 16 alpha-hydroxylation of progesterone, form 3b prepared from strain IIIVO/J exhibits a greatly diminished 6 beta-hydroxylase activity. This difference is attributable to a large difference in Km for progesterone, and similar differences are seen when the metabolism of progesterone by microsomes prepared from each rabbit source is examined. The dependence of the rate of 16 alpha-hydroxylation of progesterone and similar differences are seen when the metabolism of progesterone by microsomes prepared from each rabbit source is examined. The dependence of the rate of 16 alpha-hydroxylation of progesterone on the initial substrate concentration also differs in that most preparations from NZW rabbits contain two enzymatically distinct forms and that one of these forms is apparently absent from the preparations of P-450 3b obtained from strain IIIVO/J. The two enzymatic subforms are differentially inhibited by 16 alpha-methylprogesterone. In contrast, the rates of 4-hydroxylation of biphenyl and of the 7-hydroxylation of 2-acetylaminofluorene catalyzed by form 3b prepared from strain IIIVO/J are similar to those catalyzed by the cytochrome obtained from NZW rabbits. When the two types of P-450 3b were compared by peptide mapping, they displayed similar yet distinguishable peptide closely related yet distinct. The selective loss of 6 beta-hydroxylase activity in the preparations of P-450 3b is suggestive of the genetically determined absence of a subform of the cytochrome that catalyzes this reaction and that is normally expressed in New Zealand White rabbits.