The changes of basal glycemia and of that during the glucose and insulin tolerance tests as well as of the production of hemagglutinating antibodies in rabbits immunized with crystalline insulin (CI), chromatographically purified insulin (CPI) and waste products of the chromatographic purification of insulin (WP), both with and without complete Freund's adjuvant (CFA) were studied. Following immunization with CI and its components the glycemia increased characteristically and the course of glucose and insulin tolerance tests was changed. The peak levels of antibodies were found following immunization with WP, while the lowest ones appeared following the application of CPI. Following the immunization with a mixture of antigens with CFA higher levels of antibodies were usually observed than following the administration of antigens without adjuvant. No association was found between the titers of antibodies and glycemia values. The principal morphological change in immunized animals was found to be a partial degranulation of B-cells, often accompanied by hyperplasia of A-cells. In groups immunized with CI or CPI mixed with CFA we detected even insulitis, which was absent in the other groups. Following immunization with CI mixed with CFA such cells were found whose granules in the ultrastructure showed conspicuous similarity to those of foetal (immature) rabbit B-cells. It was assumed that in insulin immunized rabbits the initial damage of islets was followed, at least in some regions, by reparative processes. This was indicated by the findings of mitosis figures in B-cells as well as by the islet A-cell hyperplasia.