Because urinary C-reactive protein (CRP) has potential as a diagnostic marker, a sensitive enzyme immunoassay has been developed to measure this substance. This assay involves immobilizing antibody to CRP on a plastic surface, followed sequentially by addition of sample to be assayed, anti-CRP labeled with biotin, avidin conjugated with enzyme, and substrate. As little as 0.5 ng of CRP can be detected by the assay, which is reproducible and correlates well with a radial immunodiffusion method. Because the assay is inhibited by urine samples having a low pH or high urea concentration, it is necessary to pretreat the samples by solvent exchange in a commercial, disposable concentration unit. Application of the assay to clinical specimens gives results similar to those obtained previously by a complement fixation assay.