To identify candidates for feline acute phase proteins, the concentrations of serum amyloid A protein (SAA), alpha 1-acid glycoprotein (alpha 1-AG), C-reactive protein (CRP), and haptoglobin (Hp) were measured in sera isolated from clinically normal and hospitalized (or diseased) cats, from cats with experimentally induced inflammation, and cats subjected to surgery for urinary diversion. Measurements were made by sandwich enzyme-linked immunosorbent assay and single radial immunodiffusion. The concentrations of SAA, alpha 1-AG, and Hp in sera from hospitalized cats were 7-11 times higher than in clinically normal cats. Similar results were obtained for the concentrations of SAA, alpha 1-AG, and Hp in cats with induced inflammation and cats subjected to surgery. By contrast, the serum concentration of feline CRP did not change significantly between clinically normal cats and hospitalized cats or inflammation-induced or post-surgery cats. Feline SAA concentration was found to increase earliest, with alpha 1-AG and Hp beginning to increase thereafter. From these results, feline SAA is concluded to be an acute phase reactant at the early stage of inflammation.