THE P component is a normal 9.5 S serum α1 glycoprotein composed of subunits that is found as a minor constituent of all types of amyloid deposits1,2. Painter et al.3 have described a 9.5 S α1 glycoprotein, C1t, composed of ten 23,000 molecular weight (MW) subunits, which may have an important role in the regulation of complement C1 function. Ultrastructural and immunological studies showed a striking resemblance between P and C1t (refs 3, 4). CRP is an acute phase reactant, present in many species which can interact with pneumococcal C-polysaccharide, choline phosphatides and various polycations to activate the complement cascade5,6. It is composed of a 23,000 MW subunit non-covalently linked as a pentamer7. A clear-cut structural relationship between C1t and CRP has been demonstrated by comparing amino terminal sequences 7. Since all three molecules-P, C1t and CRP-have a similar ultrastructure, namely, a cyclic pentamer or a decameric double, the term 'pentraxins' has been proposed by Osmand et al. for this group of molecules 7. We report here the characterisation and the amino terminal sequence of human P component. The P component is identical to C1t except for the absence of the first residue (His), thus providing additional evidence for a close evolutionary relationship for P, C1t and CRP.