Polyfunctional hydrophilic microspheres of 125-nm diameter can be produced by copolymerization of acrylic monomers. Purified c-reactive protein (CRP) was covalently bound to these new micropheres, and the conjugate obtained was used as reagent in a microparticle-enhanced nephelometric immunoassay for human CRP. This assay was based on the measure, with a specially designed nephelometer, of the light scattered by aggregates formed during the immunoagglutination of the conjugate with anti-CRP antiserum. Sensitive inhibition of this agglutination by free CRP (6 ng/ml) allowed CRP quantitation in highly diluted serum samples (1/500-1/2,000), excluding any interference or sample pretreatment. The CRP assay was easy to perform (no washing or phase separation), reliable (coefficients of variation ranged from 1.3% to 9.3% for within-run and between-run determinations), and accurate (mean percentage of recovery: 104%; correlation coefficients with accepted analytical methods greater than or equal to 0.97) over a large range of concentrations. The inhibition mode excluded errors in the antigen excess zone and provided total security at high concentrations.