A highly sensitive radioimmunoassay was developed to measure the serum levels of SAP. Sera from 50 normal individuals ranging in age from 4 to 89 years had a mean level of 7.47 mg/dl and showed no variations with age. Sera from persons with various clinical types of amyloidosis, connective tissue diseases, and bacterial pneumonia did not differ significantly from normal values. A significant difference was noted in the sera from persons with malignancy, where a mean value of 10.79 mg/dl was determined. Although a number of similarities exist between SAP and CRP, SAP does not share the property of being an acute-phase reactant.