Some tumours respond favourably to tumour necrosis factor-alpha (TNF-alpha). Despite this preferential sensitivity, resistance to TNF-alpha remains a clinical problem and more interest is now being focused on finding compounds that induce apoptosis through other pathways. Sodium butyrate (NaBt) has anti-tumour effects on colon cancer cells, inhibiting cell growth and promoting differentiation and apoptosis. In this study we investigated whether NaBt induced apoptosis in the human colon cancer cell line HT-29 and examined the intracellular mechanisms involved. Pre-incubation of cells with NaBt significantly increased apoptosis as measured by fluorescence activated cell sorter analysis and mitochondrial membrane potential determination. This effect could be blocked with the caspase inhibitors, z-VAD-fmk (pan-caspase inhibitor), z-DEVD-fmk (caspase-3 inhibitor) and z-LEHD-fmk (caspase-9 inhibitor), but not with z-IETD-fmk (caspase-8 inhibitor). Enhancement of caspase-3 and caspase-9 activities suggests that NaBt induces apoptosis via mitochondrial pathways not involving TNF-alpha.