The serum concentration of protein SAP (amyloid P component) has been measured for the first time in a substantial series of normal individuals and patients with various diseases, and the results contrasted with the levels of the related protein C-reactive protein (CRP). The mean +/- s.d. concentration of protein SAP was 43 +/- 14 microgram/ml in seventy-six normal men, 33 +/- 10 microgram/ml in eighty-six normal women and 4 +/- 2 microgram/ml in thirty-six normal cord sera. Unlike CRP, whick is a major acute phase reactant, protein SAP was only slightly elevated in inflammatory and neoplastic diseases in which CRP was greatly increased. The level of protein SAP was significantly depressed in patients with hepatic disease, suggesting that its measurement might be of value in their management.