A total of 16,909 cultures of Streptococcus pyogenes (Lancefield group A) isolated in Britain during 1980-90 were examined for T- and M-protein antigens. One or other M antigen was detected in 92.6% of the strains. The numbers of isolates of some serotypes, such as M3 and M12, did not show great variation from year-to-year, whereas there were nation-wide epidemics, extending over several years, caused by strains of serotypes M1 and M49. Isolates of serotypes M1 and M3 were associated particularly with invasive disease and fatal infections. Representatives of serotypes M80, M81 and the provisional types PT180, PT1658 and PT5757 were isolated most often from cases of pyoderma. Erythromycin resistance was detected in 30 serotypes but one half of all of the resistant isolates belong to serotype M4.