1. 1. Channel catfish (Ictalurus punctatus) serum contains a protein that precipitates pneumococcal C-polysaccharide (CPS) in a calcium-dependent fashion. 2. 2. The serum titer of this protein follows an acute-phase pattern in catfish injected with turpentine. 3. 3. A non-glycosylated, phosphorylcholine (PC)-reactive protein (PRP) with molecular mass ca 100 kDa, was isolated from channel catfish acute-phase sera by affinity chromatography on PC-Sepharose 4B. 4. 4. Contaminating proteins with molecular masses ca 700 kDa and ca 20 kDa that co-eluted with PRP from PC-Sepharose appear to be aggregated and native low-molecular weight factors (LMFs), respectively. 5. 5. Purified PRP has γ mobility but in serum samples PRP has γ-β mobility. 6. 6. Electron microscopy confirmed that PRP has planar, pentagonal symmetry. 7. 7. The amino terminus of PRP is blocked, but based on comparison of amino-acid compositions channel catfish PRP is clearly similar to human CRP and is most like CRPs from the dogfish (Mustelus canis) and rainbow trout (Oncorhynchus mykiss). © 1992.