1. Twenty-eight normal and splenectomized rabbits were injected intravenously or intraperitoneally for from 7 to 14 courses of 4 biweekly injections with erythrocytes of BALB/c, C57B, and DBA inbred strains of mice.

  2. Antisera collected after each course of injections were examined for hemolysin and agglutinin titers as well as their ability to produce anemia in mice. Ability of the antiserum to hemolyze erythrocytes in vivo and in vitro (the hemolytic ratio) was compared.

  3. The three immunologic activities could not be detected in the serum of normal rabbits. They rose rapidly and significantly after the first two courses of injections. With few exceptions, the peaks of in vitro and in vivo reacting antibodies were reached after from three to four courses; thereafter, the in vivo activity decreased steadily, while the in vitro titers were maintained or increased. This dissociation pattern was less distinct or was absent in sera of intravenously injected splenectomized and previously immunized rabbits, and of intact rabbits injected intraperitoneally.

  4. Although the immunologic activity of antisera of rabbits injected with erythrocytes from different strains of mice was different in potency, the pattern of in vivo and in vitro dissociation was similar.

  5. Antisera of rabbits injected intraperitoneally showed much lower immunologic activity than those of intravenously injected rabbits and reached their peaks only after six courses of injections.

  6. After prolonged immunization, antisera of splenectomized rabbits showed a tendency to regain their in vivo potency while their in vitro activity was decreasing.

  7. Responses of previously immunized rabbits to a single reinjection of erythrocytes were determined by the intervals since the preceding series of immunization; they were totally unresponsive after 1 month, selectively responsive after 3 months, and showed a transient exaggerated response after 6 months.

  8. Hemolytic ratio which expresses numerically the differences in the relative activity of antisera in vivo as compared with in vitro illustrates clearly the similarities and differences in rabbits under different experimental conditions, and in different stages of immunization. It may be that differences in the distribution of cells and tissues which participate in the production of the various antibodies are a determining factor.

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