1. Blood group substances have been prepared from human saliva, stomach, and amniotic fluid from individuals of blood group A1 and A2. Several of the saliva samples were obtained from individuals shown to be heterozygous, A1O.
2. The purified blood group A substances from human sources were similar in nitrogen, glucosamine, reducing sugar, and acetyl content. The A1 and A2 substances differed in optical rotation. All of the human A samples were levorotatory while those from hog stomach were dextrorotatory.
3. By two immunochemical criteria the various human preparations could be shown to fall into distinct groups, with respect to purity. The best products showed maximal activity and almost all of their glucosamine was specifically precipitable by anti-A. These samples of human A substance were only about one-half as effective in precipitating antibody to hog A substance formed in man as was homologous hog A substance although the same total amount of antibody was precipitable by excess of either antigen.
4. Human blood group A1 substance was found to be antigenic in individuals of blood groups B and O but was not as good an antigen as hog A substance.