Sfb protein, a fibronectin binding adhesin of Streptococcus pyogenes (Lancefield group A streptococcus), mediates streptococcal adherence to human epithelial cells via its fibronectin binding domain coded by a repetitive gene region named fnbr. In the present study, Southern blot analysis using the fnbr gene region as a probe to screen genomic DNA from 51 epidemiologically unrelated clinical isolates of S. pyogenes revealed that 70% carried a sequence homologous to the fnbr probe. Among ten other streptococcal strains belonging to serological groups B, C, and G, DNA from only two human S. equisimilis (group C) strains reacted with the probe. Further analysis by PCR-mediated amplification of the binding repeat coding sequences revealed that repeats of different S. pyogenes isolates were identical in size but varied in number, ranging from one to five. Most of the isolates were shown to carry multiple repeats. Presence of the probe-positive sequence correlated strongly with streptococcal binding to purified fibronectin and adherence to HEp2 human epithelial cells; of the 36 probe-positive isolates, 95% bound fibronectin and 89% adhered strongly to epithelial cells, whereas among the 15 probe-negative isolates only 27% had binding activities for fibronectin and 27% showed strong adherence to HEp2 cells. Antibodies raised against the fibronectin binding domain of Sfb protein recognized streptococcal fibronectin binding surface proteins in most of the clinical isolates but did not react with heart or skeletal muscle myosin in an enzyme immunoassay, as is the case with antibodies directed to M protein, another major surface protein of group A streptococci. The results of the present study suggest that Sfb protein could be a potential candidate for a streptococcal vaccine.