To determine whether there are major differences in the distribution of Streptococcus pyogenes serotypes in different countries and to compare the typing results obtained in different laboratories with the same cultures, an international survey of the distribution of such serotypes was carried out in 1964-65 in 12 laboratories in different parts of the world.The cultures typed were a representative selection of those causing disease in each country, were as far as possible uninfluenced by seasonal changes and local prevalences, and included separate collections from cases of scarlet fever, sore throat, and other streptococcal diseases.Typing was carried out by the T-agglutination method in 12 laboratories and by the M-precipitation method as well in 8. Between 80% and 98% of the cultures were typable by T-agglutination in the different laboratories, and between 23% and 56% by M-precipitation.Some T-agglutination patterns were associated with a single M-antigen but others included members of several M-types. The same M-antigen was rarely found in streptococci with different T-agglutination patterns. The two methods could therefore conveniently be used as a combined system of typing.Certain streptococcal types were relatively common in most countries, but there were considerable national differences in type-distribution. Separate analysis of the cultures obtained from individual streptococcal diseases showed that there were also differences in type-distribution between streptococci isolated from scarlet fever, from sore throat, and from septic lesions not associated with the respiratory tract.