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1. Non-avid tetanus antitoxin appears to have a higher titre in relation to an avid standard if titrations are made against test toxins with a high toxoid content. The least avid serum tested appeared to be 2·87 times as potent when titrations were made against a toxin grown for 35 days than when a 2-day growth was used. Using the same test doses avid sera did not show this discrepancy.
2. A mixture of short-growth toxin and formol toxoid reacted similarly to the long-growth toxin.
3. Some non-avid tetanus antitoxic sera showed evidence of difference in goodness of fit with two test toxins; one serum was 2·49 times the titre of another when tested against one toxin and only 1·37 times when tested against another toxin.
4. Diphtheria toxoid has less affinity than toxin for antitoxin; this difference is greater with formol toxoids of poor immunizing efficiency; the difference is further accentuated if non-avid anti-toxin is used.
5. The Danysz effect may be very small if non-avid antitoxin is used.
6. In the presence of excess antitoxin, toxin or formol toxoid can combine with 3, 4 or more times its equivalent as judged by in vivo titrations for free antitoxin.
7. The serum ratio, i.e. in vivo titre/in vitro titre of high or low ratio antitoxin in multiple combinations is nearer unity than that of the original antitoxin.