Mice were immunized against challenge with Hymenolepis diminuta by feeding cysticercoids or by surgically implanting into the duodenum strobilate worms of different ages. Young worms stimulate stronger immunity than older ones, although the latter presents the host with a greater amount of strobilar tissue per unit time. An increase in the number of immunizing worms is associated with an increase in the level of protection. It is concluded that the development of functional immunity against H. diminuta in mice has both quantitative and qualitative antigenic requirements; it is influenced by worm age and is independent of worm mass.