C-reactive protein (CRP) was considered an acute-phase protein which appears only during reactions of tissue injury or inflammation. This quantitation of CRP levels in normal adults and neonates were made possible by the development of a sensitive and precise radioimmunoassay for human CRP which enables the detection of 3 ng/ml. Serum levels in 153 healthy blood donors ranged from 68-8200 ng/ml, with a median value of 580 ng/ml (mean = 1340 ng/ml). CRP levels in 24 normal cord serum samples ranged from 10-370 ng/ml, with a median value of 70 ng/ml (mean = 109 ng/ml). CRP levels in 246 individuals evaluated for autoimmune diseases ranged to 256,000 ng/ml with a median value of 13,000 ng/ml (mean = 38,000 ng/ml). No individual lacking CRP was detected. CRP can be considered a component of normal serum which only increases dramatically in concentration during inflammation.