Cysticerci of Taenia solium were exposed to gamma radiation in doses varying from 20-140 krad. Radiation had an adverse effect on the ability of the cysticerci to evaginate in vitro after a time lag of 9 days. This effect was most marked at doses of 100 krad and higher, thus no cysticerci exposed to 140, 120 and 100 krad evaginated after 12, 18 and 21 days, respectively. On Day +24, when 60% of the control cysticerci evaginated, 55%, 50%, 30% and 40% of the cysticerci exposed to 20, 40, 60, and 80 krad, respectively, evaginated in vitro. Cysticerci exposed to radiation doses of 20-120 krad are as infective to golden hamsters as are unirradiated cysticerci. Cestodes resulting from irradiated cysticerci, however, cannot maintain themselves indefinitely, and are excreted or digested at varying times from Day +12 onwards. Moreover, cestodes resulting from such irradiated cysticerci do not grow, but are resorbed, and finally consist of only a scolex. By Day +30 the mean length of the worms resulting from the unirradiated cysticerci is 173,8 mm, while those resulting from cysticerci exposed to 20 and 40 krad consist of scolices only and the hamsters fed material exposed to 60 krad were negative. It appears, therefore, that radiation inhibits the ability of the cells in the neck region to divide and thus form new proglottids. Carcasses infested with cysticercosis can possibly be rendered fit for human consumption by exposure to gamma radiation at doses between 20 and 60 krad.