Two new, rapid and sensitive radioimmunoassays for human C-reactive protein (CRP) have been established using antiserum coupled to magnetizable cellulose particles, which facilitate phase separation. A single antibody method, using solid phase anti-CRP, provides a sensitivity of 50 μg/l with a 1-h incubation time and intra- and inter-assay coefficients of variation of 10%. A double antibody method, using fluid phase rabbit anti-CRP serum and solid phase sheep anti-rabbit IgG serum, provides a sensitivity of 3 μg/l with an overnight incubation and intra- and inter-assay coefficients of variation of 10%. Among 468 sera from normal adult volunteer blood donors the median CRP concentration was 800 μg/l, interquartile range 340–1700 μg/l and range 70–29,000 μ/l. Ninety percent of samples contained less than 3 mg/1 and 99% less than 10 mg/1. Low levels (14–650 μg/l) of CRP were detected both in amniotic fluids and in cerebrospinal fluids.