A total of 378 streptococcal isolates of Lancefield groups B, C, D, and G were tested for their ability to hemagglutinate untreated, sialidase-treated, and endo- ,-galactosidase-treated human erythrocytes. Of the 43 strains showing positive hemagglutination, 9 were inhibitable with neutral monosaccharides. Four strains were inhibited with galactose and N-acetylgalactosamine, whereas five were inhibited with galactose only. A third, sialic acid-specific adhesion activity was suggested for two additional strains on the basis of their agglutination of native and endo-$-galactosidase-treated but not sialidase-treated erythrocytes. All the sugar-specific * Corresponding author.

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Irrespective of IgG Fc‐receptor activity, earlier characterized, many group A streptococci were recently found to bind aggregated IgG Fab and/or light chains. In the present study, binding of glutaraldehyde‐aggregated, radiolabelled, intact human IgG (a*IgG) to group A streptococci was tested, and strains representing several M‐types were found reactive. In particular, high binding was observed among type M12 strains, earlier found devoid of Fc‐receptors for monomeric IgG; accordingly, unlabelled, native IgG had little influence on the binding. The sites binding a*IgG were highly sensitive to trypsin and relatively resistant to heat treatment. The binding to M12 was inhibited by human fibrinogen and, to a lesser extent, by heat‐aggregated serum albumin. Rabbit antiserum to M12 was more inhibitory than antiserum to a heterologous type of group A streptococci or normal rabbit serum. Our results indicate that streptococcal M‐protein binds a*IgG by a multipoint requiring interaction of low specificity and that previously described Fc‐receptors binding native IgG are not involved. For comparison, in Cowan I staphylococci and one strain of group G streptococci tested, high binding of a*IgG was also observed; however, this binding was inhibited by native IgG, indicating that protein A and group G streptococcal Fc‐receptor, earlier known to bind untreated IgG, also bound a*IgG.

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