When bentonite particles of a selected size are "sensitized" by human gamma globulin (Fraction n) and then mixed with serum from patients with rheumatoid arthritis, they flocculate. This reaction was observed in 85% of adult patients with definite rheumatoid arthritis, in 2% of control patients, and in none of a group of normal subjects. Sera tested by the sensitized sheep erythrocyte agglutination test and the bentonite flocculation test showed agreement in 98%. Procedures for doing the bentonite flocculation test are described. The test is simple and rapid. The sera to be tested are not absorbed or fractionated.