[en] The effects of gamma irradiation of Hymenolepis diminuta cysticercoids on subsequent development into adult tapeworms were characterized. Up to 25 Krad irradiation had no demonstrable effect on the viability of the worms in mouse intestine but worm size decreased as the radiation dose increased. 35 Krad was completely effective in suppressing growth while not affecting the viability of the parasites. Survival of 35 Krad irradiated and non-irradiated worms was determined over a longer period in untreated and cortisone-treated mice. In all four groups, recovery was essentially 100% 6 days post infection. In untreated mice, non-irradiated worms had largely been expelled by the mice by 9 days post infection whereas the irradiated worm loss was not complete until day 21. Treatment with cortisone totally suppressed expulsion of irradiated and non-irradiated worms up to at least day 21. This suggests that the irradiated worms were being expelled from normal mice by a mechanism similar to that causing immune expulsion of normal H. diminuta. (U.K.)