Distribution of blood group A, B, and H(O) activities among 282 aerobic Gram-negative bacteria, many isolated from the blood of patients, has been studied. Almost half of these bacteria were found to be blood group active. About 10 per cent of the organisms exhibited high, disproportional activities, which in some instances approached those of crude human blood group mucoids. No significant, specific D (Rho), M, or N activity was found in approximately 70 members of the Enterobacteriaceae.

An attempt was made to correlate the observed activity of a given organism of known O somatic antigen with its monosaccharide components. The presence of those sugars which account for the specificity of human blood group mucoids was noted.

The bearing of these findings on the origin of human anti-A and anti-B isoantibodies has been mentioned.

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