The host–parasite relationships of Schistosoma mansoni in the albino rat have been studied recently by Ritchie, Garson & Knight (1963) and by Sadun & Bruce (1964). The general findings of these and other workers are that, in rats, the worms are always stunted, although they may pair and non-viable eggs may be found in the liver. The worms remain in the liver and seldom, if ever, migrate to the mesenteries. Ritchie et al. (1963) found that the majority of the worms were eliminated between the 4th and 8th week after infection; this is much sooner than in other experimental hosts. Sadun & Bruce (1964) suggest that rats are particularly suitable and convenient for studies on acquired resistance to S. mansoni; this suggestion is based mainly on the grounds that acquired resistance is more easily demonstrated in animals which show a relatively great degree of innate resistance.

The present work has been undertaken in order to establish whether the immune response in the albino rat is at all similar to that in the rhesus monkey (Smithers & Terry, 1965b). Two features of the host–parasite system have been studied in detail; the elimination of the worms which follows an initial infection and a comparison of the development of acquired resistance following exposure to X-irradiated cercariae with that following exposure to normal cercariae.

A Puerto-Rican strain of S. mansoni, and Sprague–Dawley rats weighing 100–200 g, were used in all experiments. The methods of exposing the rats percutaneously to infection and the recovery of the worms by perfusion have been fully described (Smithers & Terry, 1965 a).