Whang, H. Y. (Children's Hospital, Buffalo, N.Y.) and E. Neter. Immunological studies of a heterogenetic enterobacterial antigen (Kunin). J. Bacteriol. 84:1245-1250. 1962.-Study of the heterogenetic antigen revealed that it is present in relatively large amounts in supernatant fluids of heat-killed, agar-grown enterobacterial suspensions. The antigen modifies erythrocytes for agglutination by heterogenetic antibodies in Escherichia coli O14 antiserum. Of the highly purified lipopolysaccharides (10 mug/ml) tested, only a Shigella sonnei preparation had significant activity. In the hemagglutination-inhibition test, minimal activity was detected also in lipopoly-saccharides from E. coli O111 and various salmonellase. There was no parallelism in capacity of these lipopolysaccharides to neutralize heterogenetic and homologous O antibodies. The hemolytic modification of the hemagglutination test proved to be from two to eight times more sensitive than the hemagglutination method. Supernatant fluids of unheated agar-grown suspensions contained small amounts of antigen, and boiling of the bacterial suspensions resulted in the release of additional and substantial quantities. Heterogenetic antibodies did not cause agglutination of latex particles treated with heterogenetic antigen, but E. coli O14 antibodies agglutinated latex particles treated with the corresponding O antigen.