It has been reported that mitochondria-independent or mitochondria-dependent (type I/II) Fas signaling pathways in leukemia cells depend on the amount of active caspase-8. However, Bid molecules, which could not be cleaved in type I cells, could be effectively cleaved by recombinant active caspase-8 in vitro. The cleavage of recombinant Bid by recombinant active caspase-8 could be blocked by anti-p10 and anti-p18 specific antibodies. Fas receptors could be similarly internalized into cytoplasm in type I and type II cells. Interestingly, p10 subunit of active caspase-8 could be detected in both type I and II cells, while p18 subunit of active caspase-8 could be detected only in type II cells but not in type I cells. These results demonstrated that p18 subunit was necessary for Bid cleavage and the mitochondria pathway might be dependent on the release of p18 subunit from active caspase-8.