A sandwich enzyme-immunoassay (EIA) has been applied to the determination of the rheumatoid factor (RF). This non-competeitive assay comprises 3 steps: 1) the RF to be assayed is extracted for the biological medium by an immunosorbent of aggregated IgG linked to cellulose; 2) the solid phase is then incubated with the enzyme-labeled aggregated IgG; 3) the enzymatic activity of the immunosorbent is then measured with a suitable chromogenic reagent. This activity is a direct function of the amount of RF to be assayed. This assay gave reproducible results in the range 0.5-50.0 IU/ml. A good agreement was obtained between the EIA and the Waaler-Rose test but no correlation was obtained with the latex slide-test. This assay permits a quantitation of RF with a good reproducibility (coefficient of variation in the range of 10% for moderately elevated values) and thus allows a closer follow-up of patients. The results do not depend on the interpretation of the technician performing the test, which can be easily automated. Finally, it may detect some RF devoid of agglutinating activity.