Two distinct forms of cytochrome P-450 (P-450), referred to as P-450a and P-450b, were separated and purified from kidney cortex microsomes of rabbits treated with phenobarbital. P-450a had a monomeric molecular weight of 53,000, and its CO-reduced difference spectral peak was at 450 nm. It catalyzed the omega-hydroxylation of prostaglandin A1 (PGA1), and the omega- and (omega-1)-hydroxylation of myristate, but it was inactive toward exogenous compounds tested. On the other hand, P-450b had a monomeric molecular weight of 49,000, and its CO-reduced difference spectral peak was at 451 nm. This cytochrome was not able to hydroxylate PGA1 at all. It hydroxylated myristate much more slowly than P-450a, and preferentially at the (omega-1)-position. Unlike P-450a, P-450b efficiently metabolized exogenous compounds such as benzphetamine, aminopyrine, 7-ethoxycoumarin and p-nitroanisole. It is suggested that P-450a and P-450b are specialized for the metabolism of PGA1 and exogenous compounds, respectively, in kidney cortex microsomes.