1. The microquantitative precipitin method can be used to compare the relative activity of different preparations of the blood group A substance from hog stomachs and to study the effect of chemical treatment upon its stability. 2. With samples of about 25 microg. antibody nitrogen, an error of +/-1.7 microg. antibody nitrogen will result in an error of +/-12 per cent in the estimation of the amount of A substance. 3. The blood group A substance showed no significant loss of activity at 37 degrees C. after 48 hours at pH 1.07 to 10.7 or after 2 hours at 100 degrees C. over a pH range from 2.97 to 7.58. Exposure at 100 degrees C. at pH 1.03 or at 9.03 or higher resulted in loss of activity. Parallel results were obtained by the hemagglutination inhibition and quantitative precipitin methods. 4. The solubility of specific precipitates of the blood group A substance from hog stomach and its homologous antibody formed in man was found to be about 1.6 microg. antibody N/ml. 5. A comparison is given of the chemical properties and activity of blood group A substances obtained by several procedures from pools of hog stomachs.