We have extended the characterization of RLM2, a constitutive form of rat liver cytochrome P-450, using immunochemical means to quantitate its presence in microsomes, to follow its development in maturing male and female rats, and to determine its response to prototypical P-450 inducers. In addition, RLM2 is compared to RLM2b, a form of P-450 with similar migration on SDS-PAGE and NH2-terminal amino acid sequence. RLM2b is expressed in both sexes at a level of 0.08 nmol/mg microsomal protein at 2 weeks of age. In female rats, this level is unchanged with maturation. However, in the male, the level declined with maturation to reach 0.02 nmol/mg protein by 12 weeks of age. RLM2 is a male-specific form of cytochrome P-450. Originally absent in the 2-week-old rat, it reached a level of 0.03 nmol/mg protein in the adult male, its appearance and increase coinciding with the onset of puberty. Both phenobarbital and 3-methylcholanthrene induced microsomal levels of RLM2b in the adult male and female rat. RLM2, however, was suppressed in the male rat, 58 and 42%, respectively, by the same treatments. RLM2b and RLM2 each catalyze a unique spectrum of hydroxytestosterone metabolites. RLM2b is highly site specific. In contrast, RLM2 produces several isomeric products in the same region of the testosterone molecule. Substitution of the acetyl group of progesterone for the 17-hydroxy group of testosterone did not alter the site specificity of RLM2b, but did alter it for RLM2, indicating, further, a difference in the active site conformation of the two enzymes. Although RLM2b and RLM2 responded differently to inducers and to a changing physiology during maturation, and were functionally quite distinct, the proteins showed a high degree of immunologic relatedness which is suggestive of significant structural similarities. Structural differences do exist, however, as alpha-chymotryptic digestion formed a number of peptide fragments that differed between the two proteins.