To elucidate the relation between Fas-mediated apoptosis and Interleukin-1β Converting Enzyme (ICE), we examined 7 human cell lines for susceptibility to Fas-mediated apoptosis and Fas induction of ICE-like activity. The human B lymphoblastoid cell line SKW6.4 and the human T lymphoma cell lines Jurkat, CEM-6, H-9 and MOLT-4 were susceptible to Fas-mediated apoptosis whereas ICE mRNA was highly expressed in SKW6.4, H-9 and HL-60. Other ICE related proteases transcripts such as CPP32, Ich-lL and Ich-lswere detected in all 7 cell lines. An ICE specific inhibitor prevented Fas-mediated apoptosis in SKW6.4 and H-9 cells, whereas an increase of ICE-like activity during Fas-mediated apoptosis was detected only in SKW6.4 cells, but not in the other cell lines. HL-60 was unique as it expressed high levels of Fas, ICE and endogenous IL-1β, but was resistant to Fas apoptosis. To determine if IL-1β is acting as an endogenous inhibitor of ICE activity in HL-60 cells, an IL-1β expression vector was transfected into Fas-mediated apoptosis susceptible, ICE inducible SKW6.4 cells. Expression of recombinant IL-I in the Fas-mediated apoptosis susceptible SKW6.4 cells conferred partial resistance to Fas-mediated apoptosis. These results suggest that intracellular IL-1β is an endogenous inhibitor of the Fas-mediated apoptosis pathway. Activation induced cell death (AICD) through the Fas pathway is an important mechanism for down-modulation of an inflammatory response. These results indicate that a novel pro-inflammatory function for IL-1β is inhibition of AICD.