Large, independent variations occur among New Zealand White rabbits in the 21- and and 6 beta-hydroxylation of progesterone as catalysed by liver microsomes. These reactions are catalysed respectively by two electrophoretically distinct types of rabbit-liver microsomal cytochrome P-450, 1 and 3b, as judged by their catalytic efficiency and the capacity of specific monoclonal antibodies to extensively inhibit the respective microsomal hydroxylases. The relatively large variations in progesterone 6 beta-hydroxylase activity do not appear to be associated with differences in microsomal content of cytochrome P-450 3b, whereas differences in the microsomal concentration of cytochrome P-450 1 may underlie variations in 21-hydroxylase activity. Preparations of cytochrome P-450 3b contain at least two catalytically distinct subforms, one of which catalyses both 6 beta- and 16 alpha-hydroxylation of progesterone with a low Km while the other subform catalyses predominantly 16 alpha-hydroxylation with a significantly greater Km. The two catalytic subforms of cytochrome P-450 3b can be independently modulated in vitro by positive and negative effectors that can arise in vivo from the metabolism of progesterone. The 6 beta-hydroxylase subform of cytochrome P-450 3b is not expressed in a genetically defined strain of rabbits, IIIVO/J, indicating a heritable basis for the differential expression of the two subforms of cytochrome P-450 3b. These results indicate that the extent of cytochrome P-450 multiplicity may be greater than is evident from the isolation of electrophoretically distinct forms of cytochrome P-450, and that small differences in structure may underlie large differences in catalytic properties. It is not known whether the differences among outbred New Zealand White rabbits in the expression of either cytochrome P-450 1 or the subforms of cytochrome P-450 3b reflect regulatory phenomena or genetic polymorphism.