Hymenolepis microstoma grown in mice, treated with 1.25 mg cortisone acetate every second day, are heavier and have a significantly higher glycogen content than those in control mice. The dry/wet weight ratio and protein content however are unaffected. It is suggested that the increase in glycogen reserve and weight in H. microstoma from cortisone-treated mice is due to an immunosuppressive effect and better nutritional environment rather than hormonal action. Inclusion of 0.1 mg of cortisone per 100 ml of culture medium produces no change in worm weight in vitro. Single worm infections result in 100% recovery, but 30 worm infections provoke a rejection process which can be suppressed partially by administration of cortisone acetate.