We developed a new method for the determination of CRP by latex immunoassay which measures the increase in optical density as a result of latex agglutination due to antigen-antibody reaction. This latex agglutination photometric immunoassay can be used for quantitative analysis of various biological substances. We have established the best conditions for CRP determination for the instrument employed here (LA system) which allows rapid and accurate to low concentration measurement. The measurement range of the system is from 0.01-3 mg/l (non-diluted method), which when compared to the lower detection limit of RIA, 3 micrograms/l, it has the same order of magnitude. It is more sensitive than radial immunodiffusion which has the lower detection limit of 2 mg/l and has the similar value to that of radio-electroimmunodiffusion, 10 micrograms/l. Furthermore, the usual method used in the clinical laboratories, the capillary microprecipitation method, has the lower detection limit of approximately 10 mg/l. The method presented here is adequately sensitive to measure the low concentration of CRP among healthy individuals which has not been possible except by using RIA. The normal value derived from 106 healthy individuals by this method is found to be 0.2 +/- 0.2 mg/l (mean +/- 2SD).