C-reactive protein (CRP) was isolated from the acute phase serum of dogs subjected to surgical stimulation. Its properties were characterized. Canine CRP was isolated by ion-exchange chromatography using DEAE-Sephacel and DEAE-Sephadex A-50 and affinity chromatography using protein A-Sepharose CL 4B in combination with agar-block electrophoresis. In immunoelectrophoresis, canine CRP had the same gamma-mobility as human gamma-type CRP. The molecular weight of purifined canine CRP was estimated by gel filtration and sodium dodecyl sulfate polyacrylamide gel electrophoresis to be approximately 157,000 and 155,000 respectively. This CRP was a thermolabile protein which completely lost its antigenicity by heating at 70 degrees C for 15 min. The serum concentration of CRP in normal beagle dogs ranged from 0.198 to 0.826 micrograms ml-1 (0.486 +/- 0.170 micrograms ml-1). The concentration was acutely increased by surgery as it was in man and was rapidly decreased with convalescence. Dogs can be a useful animal model for investigation of the mechanism of CRP production and the function of CRP.