A series of observations on the blood of patients acutely ill with scarlet fever has shown that a toxic substance can be demonstrated in the serum by means of intracutaneous injections of the serum in persons who have not had scarlet fever and whose serums fail to blanch the rash in scarlet fever. The reaction caused by this substance consists of a bright red local erythema, varying from 20 to 70 mm. in diameter, of 1 to 4 days duration. The severer reactions are moderately indurated and tender, and are followed bypigmentation and desquamation. Control injections in persons whose serums blanch the rash in scarlet fever cause no reaction. The toxic substance is not neutralized by mixture with a human serum which gives a negative blanching test but is readily neutralized by a human serum which gives a positive blanching test. It is not neutralized by normal horse serum, but is completely neutralized by Dochez's scarlatinal antistreptococcic serum.
In a limited number of observations on the urine of patients with scarlet fever a similar toxic substance has been found in two out of five cases studied.
Since the toxic substance described appears to resemble the toxic substance found in the filtrates of scarlatinal hemolytic streptococcus cultures by Dick and Dick and since it is neutralized not only by a blanching human serum but also by Dochez's scarlatinal antistreptococcic horse serum, the experiments reported support the conception that scarlet fever is a local infection of the throat by a particular type of Streptococcus hæmolyticus capable of producing a toxin which is absorbed and is the cause of the general manifestations of the disease.