Three monoclonal antibodies, 1F11, 1G11, and 2F5, were developed toward rabbit microsomal cytochrome P-450 1. Each was linked to Sepharose in order to indirectly immunoprecipitate microsomal proteins. All three antibodies bind polypeptides from solubilized microsomes that exhibit the same electrophoretic mobility as P-450 1. However, 2F5 recognizes an additional microsomal protein that exhibits a relative electrophoretic mobility between that of P-450 1 and 3b that does not appear to be P-450 2. 1G11 also reacts with several microsomal proteins that include both P-450 1 and P-450 3b. These results indicate that at least three electrophoretically distinct forms of microsomal P-450 share one or more antigenic determinants with P-450 1. Binding studies demonstrate that each antibody can react independently with P-450 1, indicating that the three antigenic determinants recognized by the respective monoclonal antibodies are spatially distinct and nonoverlapping. Reconstituted P-450 1 isolated from rabbit liver microsomes that exhibit 10-fold higher progesterone 21-hydroxylase than most preparations of liver microsomes obtained from outbred New Zealand White rabbits catalyzes this reaction with relatively high rates compared to that exhibited by microsomes or to that catalyzed by five electrophoretically distinct forms of P-450. Both the 1F11 and 2F5 antibodies extensively inhibit the liver microsomal 21-hydroxylation of progesterone. When the 1F11 antibody is used to indirectly immunoprecipitate the antigens it recognizes in microsomes displaying a high rate of progesterone 21-hydroxylase activity, a single electrophoretic band corresponding in mobility to P-450 1 was observed.