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 µg. antibody nitrogen, an error of ±1.7 µg. 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°C. after 48 hours at pH 1.07 to 10.7 or after 2 hours at 100°C. over a pH range from 2.97 to 7.58. Exposure at 100°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 µg. 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.