The binding site interactions between the phosphorylcholine (phosphocholine)-binding mouse myeloma proteins TEPC 15, W3207, McPC 603, MOPC 167, and MOPC 511 and the isotopically substituted hapten phosphoryl[methyl-13C]choline have been investigated using 13C and 31P nuclear magnetic resonance (NMR) spectroscopy. Each protein exhibits a unique NMR pattern, but extensive similarities in chemical shift parameters upon binding of hapten to immunoglobulin suggest a significant degree of conservation of important hapten-binding site interactions. Moreover, independent binding studies, in conjunction with the NMR data, allow construction of a simple model of the binding sites of these antibodies, analyzed in terms of the relative strength of interaction between hapten and two main subsites. The NMR evidence supports the view that the heavy chains of these proteins dominate in interacting with bound phosphorylcholine; the various subspecificities of these proteins for phosphorylcholine analogues can be accounted for by amino acid changes in the hypervariable regions of the heavy chains.