Two hundred ten S. pyogenes strains isolated in 1979, 1980 and 1984 from children with pharyngitis were here examined for properties which might be relevant to their rheumatogenic potential. Strains were first tested for the production of streptococcal serum-opacity factor and, among those scored as OF-negative, the presence was then verified of M types which have been epidemiologically related to rheumatic fever. Members of "rheumatogenic" M types are present among strains causing pharyngitis in children, which, however, also include a considerable proportion of OF-positive, probably non-rheumatogenic, strains. The results are discussed in the light of the low incidence of rheumatic fever in this country.