C1r is a Ca(2+)-binding serine protease that interacts with two other plasma proteins, C1q and C1s, to form C1, the first component of the complement cascade. A monoclonal antibody, BG6, has been produced which binds to C1r only in the presence of Ca2+, requiring 3-5 microM Ca2+ for half-maximal binding. The antibody reacts with native and heat-denatured C1r, and with zymogen C1r, but does not cross-react with C1s or C1q. BG6 did not significantly affect the esterolytic activity of C1r toward a synthetic thioester substrate nor the hemolytic activity of C1 reconstituted from subcomponents in the presence of the antibody. A tryptic fragment of C1r which consists of the C-terminal gamma region of the A chain disulfide-linked to the B chain (gamma B) binds in a Ca(2+)-dependent manner to BG6-Sepharose. Western blotting experiments have further localized the epitope to the gamma region of the A chain, which is composed of two short consensus repeat (SCR) units. The N-terminal alpha region contains the only previously determined Ca(2+)-binding site in the C1r molecule. Equilibrium dialysis experiments confirmed that C1r-gamma B does not bind Ca2+, and showed that antibody BG6 and the gamma B/BG6 complex do bind Ca2+. Thus, the Ca(2+)-dependent nature of this interaction is due exclusively to binding of the metal ion to the antibody. Equilibrium dialysis and immunoblotting have further localized the Ca(2+)-binding site to the Fab fragment of BG6, indicating that the metal-induced conformational change residues in or near the variable region of the IgG. BG6 may set a precedent for the preparation of Ca(2+)-dependent antibodies to non-Ca(2+)-binding epitopes in other proteins.