Introduction. Previous to the fall of 1931 this laboratory produced diphtheria toxoid by formolizing diphtheria toxin having the usual range in potency values. The minimum values of such toxin was that set forth by the National Institute of Health, namely, of an L + value of 0.2 cc. and an m.l.d. value of 0.0025 cc., and maximum values were in the range of 25 to 50 per cent greater than these.
Following detoxification the toxoid was subjected to safety and antigenic tests in the usual manner. In addition to testing the toxoid undiluted, various dilutions were prepared with saline and subjected to antigenic and safety tests. It was noted that in some cases dilutions of 50 per cent or even 33⅓ per cent toxoid in saline possessed satisfactory antigenic qualities, namely, immunization of 8 out of 10 guinea pigs in six weeks against 5 m.l.d. of diphtheria toxin, the latter test being based on observation for ten days.
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