Summary. Serological variation in 71 oral isolates and three reference strains of Streptococcus milleri was examined. Antisera were raised by immunising rabbits with cells of 10 selected strains, followed by absorption of non-specific antibodies. Double diffusion of the typing sera and the Rantz and Randall extracts of the strains in agar gel demonstrated that 70 strains were divided into 10 serotypes (a-j) on the basis of cell-surface carbohydrate antigens. Only four strains were untypable. The typing scheme proposed depends on type antigens other than the Lancefield group antigens

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