In vitro antiparasitic effects were observed in intestinal extracts from NAMRU mice subcutaneously vaccinated with live tetrathyridia of Mesocestoides corti. All of the tetrathyridia incubated in intestinal extracts from subcutaneously vaccinated mice were dead within 8 hr and completely digested within 24 hr. Tetrathyridia incubated in intestinal extracts from mice either previously orally infected with live tetrathyridia or control mice were alive after 24 hr. A considerable number (17/20) of tetrathyridia pretreated with mouse serum from subcutaneously vaccinated mice were killed within 8 hr of incubation in intestinal extracts from untreated donor mice. The majority of the worms pretreated with either normal mouse serum or phosphate buffered saline (pH 7.2) were alive after 8 hr of incubation in intestinal extracts from untreated donor mice.