UCN-01, a selective inhibitor of protein kinase C, is known to inhibit the growth of cancer cells. Although it is currently undergoing clinical evaluation, information about its effect on human colon cancer is limited and the mechanism responsible is lacking. The objective of this study was to examine the cytotoxicity of UCN-01 to human colon cancer cellsin vitroand its effect on the apoptotic molecules. HT-29, a radiation- and chemotherapy-resistant human colon cancer cell, was used in the study. Cell death/apoptosis was determined by the MTT assay and DNA fragmentation measurement. NF-κB activity was measured by an enzyme immunoassay method. Western blot was employed to examine the expression of relevant apoptotic molecules. The result showed that UCN-01 could induce apoptosis of human colon cancer cells in a time- and dose-dependent manner. It markedly reduced the expression of Bcl-xL, but enhanced the level of p38 MAPK. In addition to Bcl-xLand p38 MAPK, UCN-01 also increased both caspase-3 and peroxisome proliferator activated receptor &ggr; protein levels. HT-29 cells transfected with exogenous Bcl-xLshowed a significant increase in NF-κB activity and prevented apoptosis induced by UCN-01. The overexpression of Bcl-xLalso reversed other relevant molecular changes observed in UCN-01-treated cells. In conclusion, UCN-01 exerted an antitumor effect in human colon cancer cells by inducing apoptosis. The mechanism responsible appeared to be related to reduction of Bcl-xLand increased p38 MAPK. The overexpression of Bcl-xLcan significantly prevent apoptosis induced by UCN-01.