1. Labile antigens in solution are spontaneously inactivated, i.e., lose their ability to combine with antibody, upon storage for a few days even at refrigerator temperature. They are inactivated by exposure to 56°C. for 30 minutes. Labile antigen has been inactivated by exposure for 10 minutes to pH 10.0 or 1 per cent neutral formalin solution. It is comparatively resistant to acid (pH 2.0).

  2. Labile antigens are soluble in 50–70 per cent ethyl alcohol. Labile antigen, prepared by disintegration of the streptococcal cells by physical means, centrifugation, filtration and precipitation at pH approximately 4.5, may be purified by solution in alcohol and recovered by rapid evaporation in vacuo. An alternative procedure for isolating labile antigen in pure state is given in detail; this procedure is in current use.

  3. Purified labile antigen from each of two strains has been investigated in the ultracentrifuge by Dr. R. W. G. Wyckoff. Each was reported to be an essentially homogeneous molecular species. The molecular weight was estimated to be of the order of magnitude of 100,000.

  4. The instability of labile antigen is in part due to its liability to undergo oxidation. Oxidized labile antigen is unable to combine with antibody. If the oxidation has been gentle, antibody-combining power may be restored by the mild reducing agents cysteine and thioglycolic acid. The presence of these reducing agents can also protect labile antigen from inactivation by oxidation.

  5. Labile antigen when treated with sodium bisulfite, is lytic for red blood corpuscles. This hemolytic power is neutralizable by antistreptolysin, by antisera against streptococci within group A or by immune-sera against labile antigen. Hemolytic labile antigen may, however, be differentiated from the oxygen-labile streptolysin of culture filtrates.

  6. A number of diverse phenomena seem then to be manifestations under various conditions of a single substance, labile antigen. Labile antigen apparently is the dominant surface antigen of virulent group A streptococci, Griffith's type-specific agglutinogen, and a compound from which group-specific and type-specific haptens are derivable. Labile antigen seems to bear a close relationship also to the oxygen-labile streptolysin of culture filtrates.

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This work has been aided by grants from the United States Public Health Service and from the Abington Memorial Hospital.

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