An immunoradiometric assay (IRMA) for C-reactive protein (CRP) was developed using magnetisable cellulose particles as the solid-phase support for anti-CRP antibodies. 125I-labeled immunopurified anti-CRP antibody was used to quantitate the amount of CRP taken up by the solid phase. Unbound label was easily and rapidly removed by decantation after sedimenting the particles on a magnet. The assay could detect 1 microgram CRP/l and had a range of up to 10 mg/l with the portion of the standard curve between 10 micrograms/l and 2-3 mg/l being linear. Fifty samples per hour could be processed manually from serum to CRP result with an intra-assay CV of 5.2% and an inter-assay CV of 10.0%, based on 5 replicates of 5 samples with CRP levels between 2 mg/l and 180 mg/l run in 5 separate assays. Fifty clinical samples were assayed in parallel with a standard electroimmunoassay and yielded a linear correlation coefficient (r) of 0.975 and a slope of 0.988. With its single, brief incubation step including all reagents and its simple phase separation procedure the present method may be the assay of choice when precise measurement of CRP concentrations is required rapidly.