On the basis of starch synthesis and degradation by the strains three subdivisions of Group A streptococci were made. In defined cultural conditions about a third of the strains accumulated starch, the majority belonging to one subgroup. Of patients from whom starch-accumulating strains were isolated at the onset of infection, the observed numbers who developed either acute nephritis or acute rheumatic fever were more than three times the expected numbers. Of eighty-six strains, including forty-six members of Type 12, and forty belonging to several other serotypes, which were associated with either nephritis or rheumatic fever, more than 90% were starch-accumulating strains. Though 47% of all Type 12 strains accumulated starch, there was a primary association of starch-accumulating strains and the sequelae, irrespective of serotype. On the basis of type-frequency, and the starch synthesizing and degrading activities of the three most common serotypes, Types 1, 4 and 12, together with the host factor (for only 25% of patients infected with starch-accumulating strains developed the sequelae) the chances of Type 12 incidents are four to one against Types 1 and 4. The probability of the findings occurring by chance was considerably less than 0·001.

Technical assistance was paid for by grants from the Royal Free Hospital Endowment Fund, and I thank the Board of Governors of the Hospital. The drawings were made by Mrs Audrey Besterman, medical artist. For all their help and patience I wish to thank Dr R. E. O. Williams, Dr M. Patricia Jevons, Mr W. R. Maxted, Dr P. J. L. Sequeira, Dr W. J. Whelan and Dr Rebecca Lancefield.