Human CRP efficiently activates the classical complement pathway via C1q. Protein SAP (amyloid P-component) is a normal plasma protein which resembles CRP structurally, but differs in not being an actue phase reactant. Both proteins can interact with cell membranes but their in vivo functions are not known; they are presumably important since, as we now report for the first time, very similar molecules are also present in the sera of diverse species including elasmobranchs, teleosts, amphibia, birds and non-primate mammals. CRP and protein SAP were isolated by calcium-dependent affinity chromatography on insolubilised pneumococcal C-polysaccharide and agarose respectively, followed by gel filtration. The different proteins all had pentagonal disc-like molecules in the electron microscope and there were marked similarities between the CRP and SAP of such diverse species as the plaice and man. The pure proteins each yielded homogeneous subunits in SDS-PAGE with molecular weights of between 20–30,000 in different species.