Congenitally athymic nude Balb/c (nu/nu) and their phenothypically normal adult and neonate littermates (nu/+), the C3H/HeN as well, were intraperitoneally infected with two strains (Y or CL) of Trypanossoma cruzi. The nude mice and the neonates developed a severe parasitaemia, the susceptible C3H/HeN also presented high level and adult Balb/c mice presented parasitaemia similar to that observed in outbred mice. Erythematous skin lesions were observed initially in infected athymic nude and neonates, being charactherized by nests of amastigotes in the dermis; in C3H/HeN infected mice no nest of parasite was observed but a low-grade inflammatory reaction was seen. In adult Balb/c or OF1 outbred mice nest was found but discreet inflammatory reaction was observed in severe infections.