Certain serotypes of group A streptococci possess the ability to produce opalescence in human or animal sera. This serum opacity reaction is mediated by a factor (OF) which acts upon the α1-lipoprotein fraction of serum. The extracellular OF elaborated by a strain of group A streptococcus M type 2 was purified by ammonium sulphate precipitation, DEAE-cellulose and hydroxylapatite column chromatography and Sephadex G-200 gel filtration. Gel filtration experiments indicated that the OF is constituted of high molecular weight proteins or protein aggregates which appear to dissociate into subunits of 66000 minimum molecular weight as determined by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The purified preparations had no group A carbohydrate or T protein antigens. Passive haemagglutination and indirect immunofluorescence tests indicated that Opacity Factor is distinct from the M type 2 protein antigen.
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