(1) When rabbits are injected intraperitoneally with ox's red blood corpuscles the haemolytic immune-body which is developed shows qualitative differences at different stages of immunisation.

(2) The immune-body molecules which appear in the serum in the early stage of immunisation (e.g. four to eight days after a single injection of 2 to 4 c.c. of red blood corpuscles) are deficient in the power of causing absorption of complement when added to the corresponding blood corpuscles. This is most clearly brought out by the very slight increase in complement absorbed under the influence of multiple doses of immune-body as compared with the amount absorbed by one dose. The deficient complement absorption does not depend to any marked degree on deficient combination of immune-body with the receptors of the red corpuscles.

(3) When, after repeated injections of blood corpuscles, immunisation has been carried to such a stage that an immune-body is produced which is very active in causing absorption of complement, then it is found that on ceasing to give further injections the relative complement-combining power remains high after the immune-body content of the serum as measured by the haemolytic dose has fallen to a very considerable extent.

So far as we are aware the occurrence of such alterations in the properties of immune-body during the process of immunisation has not hitherto been noted.