789part of a different molecule. Unpublished observations, indeed, indicate that there is a difference in CA from E. coZi 014 and from other bacteria, for CA from the former is largely insoluble and that from the latter soluble in 85% ethanol.The present observations of the inhibitory effect of lipopolysaccharides, and probably of lipoid A as well, on the CA antibody response of the rabbit appear to explain the poor immunogenicity of CA present in the bacterial cells and in crude supernates from organisms other than E. coli 014. Moreover, eventual elucidation of the mode of action of lipopolysaccharides as inhibitors of the 3CA antibody response may throw light on basic mechanisms of antibody formation.Summary. A common antigen (CA) is present in many species and serogroups of enteric bacteria, but only the antigen from E. coli 014 engenders CA antibodies in the rabbit upon intravenous injection. The ethanol soluble fraction of the former, however, induces the formation of CA antibodies. The present study has revealed that lipopolysaccharides (endotoxins) inhibit this CA antibody response. This inhibition was observed regularly when CA and lipopolysaccharides were injected as mixtures, but not when administered separately, although simultaneously, into different veins. Injection of mixtures prepared in ethanol were even less immunogenic than those prepared in water. A single preparation of lipoid A also inhibited the CA antibody response. It is postulated that the p r immunogenicity of enteric bacteria, other than E. coZi 014, is due to the inhibitory effect of the lipopolysaccharide on the fully immunogenic CA. 1. Kunin, 1C. M., Beard, M. V., Halmagyi, N. E., PROC. SOC. E n .

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