Characterization of group A beta-haemolytic streptococci in upper respiratory tract isolates from the USA and Thailand revealed that whereas 80% of the U.S. isolates could be M or opacity factor (OF) typed, less than 20% of the Thai isolates could be characterized with the available typing sera (P less than 0.001). There was also a statistically significant difference observed in the percentage of strains that could be characterized by the T-agglutination pattern (93% in the USA vs 61% in Thailand, P less than 0.001). Even among the identifiable strains, marked differences in the distribution of the recovered serotypes were noted between the two countries. These results show that there are a significant number of as yet unidentified group A streptococcal strains in parts of the world where streptococcal infections and their sequelae are important public health problems. They further imply that such findings must be taken into consideration in the future when designing possible streptococcal vaccines for worldwide use.