1. It has been verified that a single series of injections of small quantities (2 to 3 cc.) of undiluted serum-protein into rabbits will yield an antiserum that will give “group” reactions but seldom “extra-group” reactions.

  2. Such antisera do not give settled precipitates under the conditions described and the flocculative titer is lower than the interfacial titer.

  3. Reinjection of such rabbits with minute quantities of a 2 per cent homologous antigen results in an antiserum but slightly different from the initial antiserum produced.

  4. The titer of the rabbit's blood falls considerably within a 2 months' period after the first injection-series or reinjections with dilute solutions.

  5. Reinjections with undiluted serum result in antisera whose reactions (a) are aspecific, (b) give much settled precipitation, (c) react more quickly than those resulting from one injectionseries or reinjection of 2 per cent solutions, (d) reach a maximal titer (after one hour) usually after 2 injection-series, which thereafter cannot be increased by further injections.

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