Antibody to at least one type of streptococcal opacity factor (OF) was present in 39.0% of 235 selected subjects and, in 47.8% of these, to more than one type. Only 21.6% of children less than 4 years old had antibody to OF; these were to one type in 62.5% or to more than one in 37.5%. In the study group as a whole, the commonest antibodies were those to OF from M-serotype 25 followed, in descending order, by serotypes 4, 22, 2, 9, 48, 49, 28, 61 and 75. Patients with rheumatic fever or rheumatic heart disease most often had antibodies to OF from M-types 25, 22, 4 and 9, whereas in patients with acute glomerulonephritis, antibodies to OF from M-types 48, 49 and 61 were commonest. The limitations of the OF-inhibition test as an epidemiological marker for prevalent M-types of group A streptococci are discussed.


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