The effects Lactobacillus casei YIT9108 (LC 9018) on antitumor activity and cytokine production in Meth A fibrosarcoma (Meth A)-bearing BALB/c mice were examined. Intrapleural (i.pl.) administration of LC 9018 was effective in prolonging the survival of Meth A-bearing mice, and frequently cured mice of the tumor. However, the results also indicated that the effect of LC 9018 was in part inhibited in mice treated with anti-CD3 or anti-CD8 antibody, but not affected in anti-CD4 antibody-treated mice. In contrast, LC 9018 had little effect on Meth A-bearing SCID or nude mice. These results demonstrated that CD8+ T cells participated in prolonging the survival of Meth A-bearing mice. Moreover, the examination of the production of several cytokines revealed that the production of interferon-gamma and interleukin-6 was, in particular, augmented in the exudated fluid of the thoracic cavity in BALB/c mice injected with LC 9018 i.pl. These results suggested that i.pl. administration of LC 9018 induced those cytokines which had the potential to activate the thoracic macrophages or proliferate the thoracic lymphocytes to the cytotoxic T cells. Taken together, these findings demonstrated that the prolonging effects on survival by i.pl. administration of LC 9018 depended on CD8+ T cells, and the i.pl. administration of LC 9018 into i.pl. Meth A-bearing mice induced several cytokines which participated in the subsequent immunoresponses.