Monoclonal antibodies are employed as molecular probes to study human renin structure and synthesis. In this study, we observed that the active site peptide directed antibody did not bind native renin consistent with the hypothesis that the active site is at the bottom of a cleft. On the other hand, sequences that determine substrate or species specificity are on the surface of the molecule. Based on studies using antibodies directed to N-or C-teminus of prosegment of prorenin we propose that the N-terminus is hidden within the conformation of the molecule, whereas the C-terminus (activation site) is on the surface. Additional studies using monoclonal antibodies demonstrate that the human kidney contains a 50 kd prorenin, a 42 kd N-terminal truncated prorenin and a 38 kd mature renin. Our data suggest that prorenin is processed to renin by two or more sequential N-terminal cleavages.