SUMMARY: By subcultivating a streptococcal Z3 III strain in medium containing anti-III serum a strain lacking the type antigen was isolated. Evidence is given that this strain possesses only Z3 antigen. From partial acid hydrolysates of formamide extracts of both Z3 and Z3 III bacteria the same disaccharide was isolated. The most probable structure of the disaccharide is 3-O-α--acetyl-D-glucosaminoyl--acetyl-D-galactosamine. It was 250 times more active than a-methyl-JV-acetylglucosamine in the inhibition of the Z3/anti-z3 system. This denotes that it is an important part of the determinant group of the Z3 antigen. By ethanol fractionation of a formamide extract of Z3III bacteria two distinct fractions were isolated. The first fraction reacted only with type III antiserum and consisted of glucose, galactose and rhamnose in the ratio 5:3:1. The second polymer was composed of rhaninose, glucosamine and galactosamine in relative amounts of about 2:1:1. Chemical and serological evidence suggests very strongly that this is the Z3 antigen. The similarities between z and true group antigens are discussed.


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