THE intradermal injection of a small quantity of diphtheria toxin into a guinea-pig is followed by a definite inflammatory area round the point of injection. This red area is always visible within 36 hours after the injection and often may be seen within 18 hours and occasionally in less than 8 hours after injection. The time of appearance of the reaction is no indication of the time elapsing between the injection and the fixation of the toxin by the living tissues. The time of fixation is of both practical and theoretical interest.Most of the work in the past on the time of fixation of diphtheria toxin has been done by subcutaneous injection, but Schick (1923) when working out the method of diagnosis of susceptibility to diphtheria to which his name is attached injected intracutaneously a number of children with a diagnostic dose of diphtheria, toxin followed later by a subcutaneous injection of antitoxin. He found that the size of the reaction was unaffected on every occasion when antitoxin was given 9 hours after the injection of toxin; usually, the injection of toxin caused a full-sized reaction if 6 hours elapsed before antitoxin was given, and a reduction in size of reaction nearly always resulted if antitoxin was given 3 hours later. I n the experiments recorded in this paper guinea-pigs were injected intracutaneously with a series of Schick doses of toxin a t varying intervals of time and antitoxin was administered during or after the series of injections of toxin. I n the majority of cases, the antitoxin was injected intravenously to ensure immediate distribution.It is possible, from a consideration of the reactions produced, to decide in what time an amount of toxin injected is so completely fixed by the tissues that no amount of antitoxin given a t the end of that time can affect the action of the toxin, i.e. the size of the reaction produced. A t the other end of the scale one can find how long one can wait after the injection of an amount of toxin and yet be certain that notwithstanding the delay, one can inject antitoxin and so "catch u p " the action of the toxin and ensure the suppression of any reaction. If the interval of time elapsing between JOURN. OF PATH.-VOL.
XXVIlI.
R
