* T.t. e = mouse strain; T.t. m = rat strain. similar way Wistar rats were infected with 100 T.t. e and challenged with 30 T.t. m eggs. In both experiments mice and rats (divided at random with the challenged animals) were used as control of egg viability. The infection was carried out following the technique described elsewhere (Ferretti et al., 1980, Boll. Zool. 47: 165-184). The experiment was repeated three times with the same results. The results of one of these experiments are shown in Table I and are such that statistical analysis is superfluous. Infection with an unsuitable strain gives complete protection against the suitable strain. The hypothesis could be put forward that an analogous protection may exist among the various strains of E. granulosus. This work was supported by a Grant from the Assessorato alla Difesa dell'Ambiente, Regione Autonoma della Sardegna. ilar way Wistar rats w re infected with 100