1. Certain strains of lactose and non-lactose-fermenting coliform bacilli have been found to possess a common agglutinogen. This antigen is found in both motile and non-motile smooth strains and appears to be distinct from ‘H’, ‘O’ and rough antigens, and from the X antigen described by Topley & Ayrton. In certain respects it seems to resemble the Vi antigen of Felix & Pitt, notably in the inhibitory effect it exerts on ‘O’ agglutination. It is, however, not associated with virulence and is of relatively wide distribution.
2. The strains, when inoculated into rabbits, stimulate the production to high titre of an agglutinin which can be absorbed out completely by any one of the strains.
3. Subculture of the strains under certain conditions results in the loss of the antigen and the development of specific variants.
4. Agglutinins to these strains have been found in certain diagnostic sera and may prove a possible source of error, particularly if slide agglutination is relied on for identification.
We take this opportunity of expressing our thanks to Prof. G. S. Wilson for the loan of Bact. typhi-murium ‘X’ antiserum.