Streptococcus pyogenes is a major human pathogen. It is a common cause of pharyngitis, cellulitis and wound infections. Late complications like rheumatoid arthritis and glomerulonephritis are associated with certain M proteins on the surface of the bacteria. In 1987 an increase was noted in the incidence of serious infections caused by this bacterium. The increase has been associated with protein type M 1. Typing with antibodies against T proteins is simpler to perform than M typing and can give as good epidemiological information. Culture results from January 11986 to December 31 1993, from the Department of Microbiology at the National University Hospital in Reykjavik, were reviewed. T protein type of some of the strains, that had been preserved by freezing, was determined by agglutination after culture in Todd Hewitt broth as described by Efstratiou. T-protein type of 384 strains from 1991-1993 was determined and the results compared to unpublished results from 1988 and 1989. T-protein type was also determined on all S. pyogenes strains that were isolated from blood in 1989 to 1993. The following T-types were most common: 1988-1989 Tl vas 30%; 1991, T4 and T28 70% and 62% in 1992; in 1993 Tl and T3 were 59%. Thirty one strains were sent to the Streptococcal Reference Laboratory, Central Public Health Laboratory, London, for determination of M-proteins. All strains but one, that were sent to Britain for M-protein typing, had corresponding T-proteins (Mx=Tx; My=Ty and so on). Big fluctuations in the number of isolations of S. pyogenes strains was observed during the study period: Fewest in 1989 or 629, but the number was highest in 1993 or 2057. The changes in incidence seemed to correlate with certain serotypes.