The occurrence of apoptosis in thyroid follicular cells induced by Fas activation has been a subject of much debate. This is due, in part, to the fact that no physiologically relevant treatment conditions have been reported to cause rapid and extensive Fas-mediated apoptosis in thyroid cells, whereas treatment with the protein synthesis inhibitor cycloheximide prior to Fas activation allows for massive cell death. This indicates that the Fas signaling pathway is present but that its function is blocked in the overwhelming majority of cultured thyroid cells. To reconcile the conflicting reports, we set out to identify physiologically relevant conditions in which rapid, massive thyroid cell apoptosis in response to Fas activation could be demonstrated. We determined that susceptibility to Fas-activated apoptosis could be influenced by certain combinations of inflammatory cytokines. Although no single cytokine was effective, pretreatment of thyroid cells with the combination of gamma-interferon and either tumor necrosis factor-alpha or interleukin 1beta allowed for massive Fas-mediated apoptosis. Susceptibility to Fas-induced death correlated with an increase in expression of a tunicamycin-inhibitable high molecular weight form of Fas but not with aggregate expression of Fas.