This study has confirmed cortisone suppression of innate resistance to Trichinella spiralis in Chinese hamsters. Recovery of carcass larvae from steroid-treated animals averaged 63,162 worms after 30 to 35 days of infection. Untreated controls yielded an average of 28 larvae from animals which had received comparable infective doses. Treatment with ACTH did not substantially alter the resistance of Chinese hamsters to the establishment of larvae in muscle tissue. The golden hamster, which exhibits virtually no innate resistance to T. spiralis, was studied comparatively. Steroid-treated golden hamsters became severely emaciated and developed secondary infections. Half of these animals failed to survive the experiment. Although recovery of total carcass larvae from the surviving steroid-treated hamsters (avg 94,168) was not substantially greater than that from untreated controls (avg 85,359), the experimental group showed larger numbers of immature muscle larvae and persistence of adult worms in the gut. In the golden hamster, treatment with ACTH ameliorated the outward signs of the infection (i.e., facial edema, etc.). The average yield of total carcass larvae from the hormonetreated group was 64,911 worms, vs. an average of 85,359 worms from untreated controls. Owing to considerable overlapping of worm counts from the latter two groups, the difference was not considered significant. With cortisone treatment, both species of hamsters seemed to harbor adult worms for a longer time than controls or ACTH-treated groups. Both cortisone (Rosen, 1952) and adrenocorticotrophic hormone (ACTH) (Luongo et al., 1951) ameliorate clinical trichinosis in man. Experimentally, using survival time and mortality as indices, others have shown that the administration of ACTH to rats (Lord, 1958), and guinea pigs (Luongo et al., loc. cit.) has a favorable effect on the host infected with Trichinella spiralis. On the other hand, Coker (1955) has shown that cortisone suppresses resistance to both intestinal and muscle phases of trichinella in mice. Zaiman et al. (1964) showed that either cortisone or ACTH decreased survival times of heavily infected mice, and Davis and Read (1958) found an increased survival time of adult worms and an enhanced yield of carcass larvae in mice subjected to behavioral stress. The latter effect was presumably mediated via the adrenal-pituitary stress syndrome. The seemingly paradoxical effects of clinical improvement vs. suppression of resistance by steroid, ACTH, and the stress syndrome merit Received for publication 2 April 1968. * This investigation was supported in part by a research grant (AI01042) from the National Microbiological Institute of the NIH, U. S. Public Health Service. t NSF Summer Fellowship (postdoctoral). Present address: St. John's University, Biology Department, Jamaica, New York. further experimental analysis. In the following study, the effects of cortisone and ACTH on trichinella infections have been compared in two species of hamsters: the golden (Mesocricetus auratus), which is exquisitely suited for the development of larvae in muscle tissue (Ritterson, 1959), and the Chinese hamster (Cricetulus griseus), which is highly resistant to the development of larvae in muscle tissue (Ritterson, 1957, 1959). Ritterson (1959) has already shown that innate resistance in the Chinese hamster is suppressed by cortisone. MATERIALS AND METHODS The Chinese hamsters used in this study were bred and raised in this laboratory; golden hamsters were obtained commercially. All animals were 3 to 4 months of age at the beginning of the experiment and except where indicated (Table I) male animals were used. All hamsters were maintained and infected with Trichinella spiralis as previously described (Ritterson, 1959). Sterile cortisone acetate (Cortone, Merck) was administered daily by subcutaneous injections of 0.6 mg in a volume of 0.05 ml. Injections were begun on the day of infection and continued through the 28th day. ACTH (ACTHAR, Armour) was diluted with sterile saline, dispensed in small vials, and frozen until used. In this way, only freshly thawed hormone was used for treatment. ACTH was administered daily (s.c.) as a divided dose consisting of a total of one unit in a volume of 0.1 ml (Table I). In a second experiment (Table II), the hormone was given as a single daily (s.c.) dose of one unit in a volume of 0.1 ml.