Using the standard agar dilution method we studied the prevalence of susceptibility to 14-, 15- and 16-membered ring macrolides and clindamycin in Streptococcus pyogenes isolated in 1998 from 21 laboratories in Spain. The number of strains admitted to the study was proportional to the numbers of inhabitants in each geographical area. We also determined the susceptibility phenotypes and the genetic basis for the antibiotic resistance. A total of 486 unduplicated isolates of S. pyogenes were used. Throat swab samples provided 359 (73.9%) isolates, and the remaining 127 isolates were from other sources. One hundred and fourteen (23.5%) isolates were resistant to erythromycin, a 14-membered ring macrolide, and azithromycin, a 15-membered macrolide, whereas only 1% of isolates were resistant to miocamycin, a 16-membered macrolide and 0.8% were resistant to clindamycin. Of the 114 erythromycin-resistant strains, 109 (95.6%) were susceptible to clindamycin and miocamycin. Since induction with erythromycin did not modify susceptibility to the latter antibiotics, these 109 strains were considered to have the M phenotype. Twenty strains with the M phenotype, one per laboratory, were assayed by PCR and showed the presence of the mef gene, which is responsible for antibiotic resistance by an efflux system. Among comparable studies covering entire countries, ours demonstrates one of the highest rates of S. pyogenes erythromycin resistance and clindamycin and miocamycin susceptibility in the world. Strains with the M phenotype account for the great majority of these isolates.