The effects of diets containing 8% and 4% protein and of 50% calorie-deficient diets on the host, the parasite, and the parameters of disease in chronic hepatosplenic schistosomiasis were studied in mice that were infected for 20 weeks with Schistosoma mansoni. Controls consisted of uninfected mice of the same size and age maintained on a balanced 20% protein diet. Each of four corresponding dietary groups of infected and uninfected animals was fed on its assigned diet from the 20th to the 24th week of infection. Between the 24th and the 28th week of infection, all groups were nutritionally repleted by being returned to the standard control diet. Changes in body weight and deaths were recorded throughout the experiment (20-28 weeks). Parasite and host responses were measured at 24 weeks and again at 28 weeks. Severe protein and calorie deficiencies were associated with diminished egg output, an effect that appeared to be residual in the most severely malnourished animals for some time after malnutrition has been corrected. In general, the parameters of hepatosplenic disease were less severe in the more severely malnourished animals; 50% calorie deficiency was the cause of greater amelioration than marked protein depletion. During the period of malnutrition the mortality rate in mice given the 4% protein diet was not significantly different from that in controls (approximately 30%), while that in the calorie-restricted animals was only 9%.