A new rat serum protein has been isolated by affinity chromatography using ethanolamine- or phosphoethanolamine-substituted agarose gels. This protein has the morphological and functional characteristics of serum amyloid P-component and C-reactive protein. It comprises C5 cyclic symmetry structure with non covalently cross-linked subunits which have calcium-dependent binding sites. We have called this protein rat serum amyloid P-component since it has all the properties typical of human serum amyloid P-component: it is made up to 10 subunits, it contains sialic acid and hexoses, it forms macroscopic polymers and its does not precipitate with pneumococcal C-polysaccharide. Rat amyloid P-component has three remarkable properties. Electron microscopy has shown that apart from pentagonal figures and stacked discs, rat P-component has a C10 cyclic symmetry structure. Rat amyloid P-component has an affinity for specific ligands, such as phosphorylcholine or phosphoethanolamine. These ligands are able to depolymerize self-associated rat P-component. With these characteristics, rat serum amyloid P-component could prove to be an important model in the study of the relations between amyloid P-component and amyloidosis.