Short-term stimulation (i.e. <2 days) with tumor necrosis factor-alpha (TNF-alpha) or interferon-gamma (IFN-gamma) cause growth arrest and sensitize epithelial cells to CD95 (Fas/Apo-1)-mediated cell death. The effect of long-term cytokine exposure on viability, proliferation, and apoptosis response of colonic epithelial cells is unknown and addressed in this study. In the present study HT29 and DLD-1 colonic cells were stimulated with either TNF-alpha or IFN-gamma at varying concentrations for 2-9 days. Viability and proliferation was assessed. CD95-mediated cell death response was determined. IFN-gamma caused decreased viability at high concentrations (1 nM), whereas lower concentrations (10-100 pM) only caused a transient growth arrest. TNF-alpha (100 pM) did not affect cell growth. Cells stimulated for 8 days with IFN-gamma (10 pM) or TNF-alpha (100 pM) had higher proliferation rates than controls or cells stimulated for 2 days (p < 0.05). Whereas the spontaneous cell death increased slightly during continuous cytokine exposure the CD95L response decreased (P < 0.01). Colonic cells continuously exposed to IFN-gamma or TNF-alpha had cell turnover characteristics that resemble findings in patients with UC. Increased proliferation and decreased cell death response may act as a counter regulatory mechanism that limits the damaging effects of cytokines.