Parasite-specific humoral and cell-mediated immune responses were investigated in highly susceptible (AKR and C57B1/6J) and relatively resistant (C57B1/10) mice undergoing secondary alveolar echinococcosis (infection with Echinococcus multilocularis metacestode). The parasite-specific proliferative immune response of lymph node cells upon in vitro antigen stimulation remained weak in all three mouse strains. By day 30 p.i., CD4+ lymphoblast cells dominated the total population of blast cells in all three mouse strains. There was, however, an unexpectedly high proportion of CD8+ blast cells; by day 90 p.i., a marked proportional increase in CD8+ cells was seen in susceptible (AKR and C57B1/6J), but not in resistant (C57B1/10) mice. Susceptible, but not resistant mice exhibited a significantly decreased responsiveness of lymph node cells to concanavalin A (Con A) stimulation on day 90 p.i. Analysis of the humoral immune response by ELISA showed that resistance in C57B1/10 mice was associated with the ability of the host to synthesize antibodies to Em2 of the IgG3 and IgG1 isotype. Em2 is a lectin-binding carbohydrate antigen of the laminated layer. In susceptible AKR and C57B1/6J mice, low levels of anti-Em2 antibodies of the IgG2a isotype were detected. Anti-Em2 antibodies of the IgG3/IgG1 isotype, however, were absent. Differences in subclass-specific IgG responses were confirmed by immunoblot analyses. Our findings suggest that differences in antigen recognition (with respect to subsets of humoral and cellular immune components), probably controlled by non-H-2 gene(s), coupled to immune suppression modulated by CD8+ cells and/or respective cytokines, may determine susceptibility or resistance in experimental infection with E. multilocularis.