The susceptibility of golden hamsters to infestation with Taenia solium is increased by treatment with antilymphocytic scrum (ALS) or whole body irradiation or chemical immunosuppressants, viz. methylprednisolone acetate and betamethazone disodium phosphate. Although the worms develop to sexual maturity they do not produce eggs. Such treatment is less effective in increasing their susceptibility to T. saginata. When young hamsters were exposed to whole body irradiation and subsequently treated with ALS, 60 per cent of them became infested with T. saginata. As young animals do not survive immunosuppression for any length of time the further development of this tapeworm was not investigated.