Three out of four anti-C-reactive protein monoclonal antibodies [HB3-2, (μ, κ)], EA4-1 (γ2a, κ) and FB2-1 (γ1, κ bind to C-reactive protein in the presence of 2.5 mMCa2+ but not in the presence of 1.0 mM EDTA. indicating that the conformation of the antigenic determinant(s) recognized by these three antibodies is dependent upon Ca2+. This Ca2+-dependent binding can be inhibited by 1.0 mM phosphocholine, indicating that this antigenic determinant is at or near the phosphocholine-binding site of C-reactive protein. The binding of the fourth monoclonal antibody [HD2-4 (γ2a, κ)] is independent of the presence of Ca2+ and is not inhibited by phosphocholine. HB3-2 (μ, κ) recognizes an antigenic determinant on the structurally related proteins, rabbit CRP and serum amyloid P. © 1982.