Synopses The Lancefield group A streptococci (Strepto-

coccus pyogenes), major causative agents of human disease (1), can produce both mild (e.g., pharyngitis) and severe (e.g., life-threatening toxic shock-like syndrome, necrotizing fasciitis) infections. During the last few years, erythromy-cin-resistant S. pyogenes (ErR) has been reported in different parts of the world (2-4). Two distinct mechanisms of erythromycin resistance are described among group A streptococci. One consists of target-site modification by erm methylase (5,6) strains that express the MLSb phenotype of resistance; the other (recently described) consists of an active drug efflux that pumps 14- and 15-membered macrolides out of the cell (7). This novel mechanism of macrolide resistance is encoded by the gene mefA (8), and strains show the M phenotype. In the last few years, increased resistance to erythromycin in S. pyogenes has been detected in Spain (9-11). Therefore, we performed an epidemiologic investigation to determine the biotypes, sero-types (T-agglutination patterns), emm types, and pulsed-field gel electrophoresis (PFGE) patterns of chromosomal DNA and their relationship to macrolide resistance