The relation between activation of caspase-8 and polyglutamine aggregates has been focused. We prepared an antiserum (anti-m8D387) that recognizes the active form but not the preform of mouse caspase-8. We used immunostaining with anti-m8D387 antiserum to compare the localizations of activated mcaspase-8 in L929 (clone 1422) cells induced by TNF and polyglutamine aggregates. Anti-m8D387 was positive throughout cytoplasm of the TUNEL-positive cells induced by TNF treatment, whereas the anti-m8D387 reactivity was not positive throughout cytoplasm of the cells expressing polyglutamine but was restricted to polyglutamine aggregates. In contrast with TNF-treated cells, cells expressing anti-m8D387-positive cytoplasmic polyglutamine aggregates did not undergo TUNEL-positive apoptosis. Thus activated caspase-8 associated with polyglutamine aggregates alone was not sufficient to induce TUNEL-positive apoptosis of L929 (clone 1422) cells. The distribution of activated caspase-8 associated with polyglutamine aggregates may be essential for the polyglutamine-mediated cell death or downstream of caspase-8 may be different in the TNF-treated cells and cells expressing polyglutamine.