Addition of protoscoleces (PSC) of Echinococcus multiocularis suppressed proliferative responses in spleen cells stimulated with concanavalin A (Con A) on day 3 of culture. The suppression was not observed in the spleen cell population depleted of CD8+ cells but observed in the population depleted of CD4+ cells, suggesting the involvement of the CD8+ T cells in the apparent suppression. Indeed, flow cytometry analysis revealed that the proportion of CD8+ cells markedly increased in the Con A cultures containing PSC. Furthermore, when spleen cells were co-cultured with PSC alone, marked increase in the proportion of CD8+ cells as well as B220+ cells was observed. Addition of these increasing CD8+ cells suppressed the proliferative responses of fresh spleen cells stimulated with Con A. These findings suggest that the low responsiveness of spleen cells to Con A on day 3 of culture in the presence of PSC is attributable to an active suppressor mechanism including CD8+ T-suppressor cells generated by stimulation with PSC.