Artificial immunization of rats against Taenia taeniaeformis was studied using somatic antigen (Som-Ag) and excretory-secretory antigen (ES-Ag). It was found that both Som-Ag and ES-Ag stimulated immediate-type hypersensitivity and delayed-type hypersensitivity reactions of similar levels. Antibody levels rose from the 2nd wk and peaked around the 6th and 7th wk. Both IgM and IgG were detectable from the 2nd wk onwards, with IgG at a considerably higher level compared to IgM. It terms of protection, 90-100% reduction in cyst counts were detected if the rats were challenged 10 days or more after immunization. In all cases, no significant difference was observed between immunization with either Som-Ag or ES-Ag were purified and characterized using Sephadex G-200 chromatography, double immunodiffusion, and disk acrylamide gel electrophoresis. A purified antigen (mol wt, 140,000 daltons) was obtained, and highly significant protection against infection resulted with injections of 50, 10, or 1 mug doses of this antigen with complete Freund's adjuvant.