Cross-resislance to various parasites in Ihe one host has been investigated mainly by two approaches: fl) conciirrent infection with iwo or more parasites, and (2) immunization with one species followed by a challenge infection wilh a heterologous species. In both situations muiuiil interactions between the parasites, and the parasites and host, seem to be complicated (reviewed by Kazacos, t975).Worms of Nippostrongylus hrasilien.sis and Strongyloides ratti both parasitize the upper 1/3 of the small intestine approximately 3 days subcutaneous inoculation of rats with infective larvae. After a plateau phase of infection (up to approximately 10 days for N. bra,^ilifn.\is and 20 days for 5. ratti), adult worms are expelled by the host's immune defence mechanism. Thereafter, the host rats are highly resistant against re-infection with homologous species (Ogilvie and Jones, 1971; Moqbel and Denham. 1977). From these fundamental similarities of the life-cycles, these two parasites would seem to be useful for analysing the specificity of the host's defence mechanism. Using N. hra.siliensis and S. ratti. Kazacos and Thorson (1975) first reported that, afler an infection with one species, rats became resistant against subsequent infection with the other. Recently, we demonstrated that this cross-resistance could be generated 'uy, and directed against, the tissue migrating larval stage but not the intestinal adult stage (Nawa et al. 1982). In N. brasitien.us-'immunizeii rals. challenged N. brasitiensi.v larvae reach the small intestine but are immediately expelled before they become fully mature adults (Love, Kelly and Dineen, 1974;Miller, Huntley and Wallace. 1981). Thus, it was of interest to see whether such a deteriorative effect of the host's immune system directed against N. brasilien.iis in the small intestine has any significant effect on concurrently existing . V. ratti.In the present study outbrcd female Wistar rats (8 weeks old and weighing approximately 200 g) were first infected with 3,000 N. brasiUensis larvae and, 4 weeks later, challenged concurrently with 1.500 A'. brasiUensis larvae and 1.500 . V. ratti larvae. As a control, normal rats were given only the concurrent infection with both species at the same dose. The degree of protection against these parasites was monitored by daily mass faecal egg counts (FEC) of each group of 5 rats.As shown in Fig.l, the FEC of A^. hrasiliensis and S. ratti in normal rats concurrently infected were similar to those observed in the single infection of individual parasites (data not shown). In N. brasilien.si.'i-immum7.ed rats concurrently infected with N. hrasiliensis and S ratti. eggs of N. brasiUensis could never be detected and the FEC of S. ratti in these rats decreased considerably during the period between days 6 and 15, though the duration of faecal egg output of S. ratti was equal to that observed in non-immunized rats. The degree of decrease in the FEC of concurrently infected . V. ratti in N. /trtj.TiV/rfrv/v-immunized rats was far greater than that reported ...