SummaryCBA mice which had been deprived of their T cells by a combination of adult thymectomy and injection of rabbit anti‐mouse thymocyte serum failed to excrete as many Schistosoma mansoni eggs in their faeces as immunologically intact controls. This failure of parasite egg excretion was not obviously attributable to any marked change in the amount of faecal matter produced, or to a change in the size of the worm burden, or to the number or distribution of eggs in the tissues as a result of T‐cell deprivation. S. mansoni eggs freshly isolated from T‐cell‐deprived mice and injected intravenously into normal animals, induced lung granulomas which were the same size as those induced by injection of eggs from normal donors. The rate of S. mansoni egg excretion was not affected by the density of eggs in the tissues, in as much as there was a linear relationship between the number of tissue‐bound eggs and the number of eggs detected in the faeces. Treatment of infected mice with the immunosuppressant hydrocortisone acetate also induced a marked reduction in the rate of egg excretion. Injections of serum derived from chronically infected normal mouse donors increased the rate of egg excretion in both T‐cell‐deprived and steroid‐treated mice, but the degree of reconstitution obtained by daily serum injections was only partial relative to normal egg excretion rates. Treatment of infected normal or deprived serum‐treated mice with cobra venom factor to reduce serum complement C3 levels had no effect on the rate of S. mans