Citrated human type O erythrocytes were treated with neutral formalin in 0.85% NaCl and were stored at 2-5[degree]C in this medium for over one year. Following removal of the formalin the erythrocytes were sensitized with recrystallized jackbean urease at pH 3.9-4.1 1 in a citrate buffer prepared by combining 0.5 [image] sodium citrate with 0.5 [image] citric acid in a ratio of 2.0:1.7. Erythrocytes in a 10% suspension were coated in a buffered medium containing crystalline urease (0.04-0.05 mg/ml), 0.4 ml NaCl, and 0.2 ml of citrate buffer per ml of suspension. The resultant mixture was rotated (60 rpm) in a small polyethylene bottle for 5 days at room temperature. Following washing in 0.85% NaCl the cell concentration was adjusted to 2.5% with 0.85% NaCl containing 0.005 [image] dibasic sodium phosphate and 1% bovine serum albumin. Hemagglutination reactions with antisera from rats, rabbits and mice immunized by different dosing schedules gave highly reproducible results which correlated well with precipitin titers. Reactions were carried out in hemagglutination trays incubated at 2-5[degree]C for 12-18 hours. The technique has given reproducible results on sera, fecal extracts and urine. Chicken erythrocytes were found necessary for antibody studies in chicks.