Activated embryos of Taenia taeniaeformis and T. ovis placed in filtration membrane diffusion chambers and implanted intraperitoneally in rats and lambs, respectively, for 3 weeks, developed at rates comparable with those reported in natural infections. The developing embryos of both cestodes induced a high degree of immunity to subsequent oral challenge in both rats and lambs. In the case of T. taeniaeformis, this immunity was greater after 3 weeks of in vivo growth than after either 1 or 2 weeks. T. ovis produced a solid immunity after only 1 week in vivo. Four main conclusions were drawn from the experiments. First, developing embryos released diffusible antigens capable of stimulating immunity in their host; direct contact between host cells and parasites was not necessary. Second, effective immunizing antigens are produced early in the development of cestode larvae. Third, sufficient antigen to immunize the host is produced by only small numbers of larvae. Fourth, the antigens are effective in immunizing the host via parenteral administration; an intestinal migration is not necessary to induce immunity to the establishment of an oral challenge infection. The relevance of these findings to the production of a vaccine or of improved antigens for diagnosis of cestode infections in man and animals is discussed. Immunity to taeniid cestode larvae has been induced experimentally in a variety of animals by means of infection with live organisms (Miller, 1931a; Sweatman, 1957; Froyd and Round, 1960), by the inoculation of live parasites into abnormal sites (Gemmell, 1962, 1964), and by the injection of extracts and suspensions of killed parasites (Miller, 1931b; Campbell, 1936). Each of these methods suffer from some disadvantages. In natural infections it is virtually impossible to control the period of exposure to antigen and it is often difficult to distinguish between parasites of the immunizing and test infections. It can be difficult also to distinguish between pathology associated with either of the two infections. When live organisms are injected into abnormal sites in the host, some may escape from these sites and develop in other parts of the body (Gemmell, 1964). Killed antigen preparations may be lacking in certain metabolic excretions or secretions (ES antigens) produced by live parasites; there is some evidence that these substances may play a part in stimulating immunity (Silverman, 1955). Filtration membrane diffusion chambers have Received for publication 8 September 1970. been used for in vivo cultivation of the nematode Trichinella spiralis in rats; partial development of this parasite occurred in intraperitoneally implanted diffusion chambers, and some degree of immunity developed in the host (Despommier and Wostmann, 1969a, b). Providing that the organisms survive, and preferably also develop, in diffusion chambers implanted into the host they provide a useful method for studying both growth of the parasite and the development of immunity in the host animal. Diffusion chambers allow strict control of the period of exposure to antigen, the host is exposed to a wide range of ES antigens, and there are no pathological changes in the host due to the immunizing infection. The present investigation was carried out to determine the suitability of diffusion chambers for studies on the growth and development of cestode larvae and the immune response of the intermediate host. MATERIALS AND METHODS