We examined an instrument-free test for C-reactive protein (CRP) in whole blood. The NycoCard CRP Whole Blood test uses a cell-solubilizing dilution liquid, a membrane-bound antibody that binds CRP, and a gold-conjugated antibody for making visible the bound CRP. We obtained essentially identical dose-response curves in citrate-, heparin-, and EDTA-treated blood. CVs were 6.7-12.5% within series and 10.1-14.7% between series. The detection limit was 12 mg/L. Intralipid added to blood increased measured CRP by 10-20%, whereas no change was seen with added bilirubin, added serum amyloid P component, or the presence of rheumatoid factor. In 234 patients' blood samples the results of the NycoCard Whole Blood test correlated well (r = 0.96) with those of a turbidimetric serum method. The test allows reliable measurement of CRP from a small volume of whole blood (25 microL) without using expensive equipment; it should be useful for decentralized testing in hospital departments, emergency units, and primary health care centers.