The broad-range cyclin-dependent kinase inhibitor 7-hydroxystaurosporine (UCN-01) is known to induce both a G1 cell cycle arrest and apoptosis. The mechanism of UCN-01-induced apoptosis is largely unknown. We analysed the mechanism of cytotoxicity of UCN-01 in four established colon carcinoma cell lines. The cell lines SW48 and LS513 responded to UCN-01 treatment by undergoing apoptosis in a concentration-dependent manner while the cell lines HT-29 and WiDr were completely resistant. Apoptosis in LS513 and SW48 cell lines was concomitant with the suppression of Bcl-x(L) on mRNA and protein level. In contrast, in the apoptosis-resistant cell lines, Bcl-x(L) expression was not affected by UCN-01. Stable overexpression of the Bcl-x(L) protein abrogated UCN-01-triggered apoptosis, but only partially restored growth, indicating that both cell cycle arrest and apoptosis exert the anticancer effect in a coordinated manner. The inhibition of Akt phosphorylation did not correlate with the apoptotic phenotype. UCN-01 inhibited the activating STAT3 phosphorylations on Ser727 and, notably, on Tyr705, but STAT3 did not contribute to Bcl-x(L) expression in colon carcinoma cells. Moreover, we show for the first time that UCN-01 induces apoptosis by suppression of Bcl-x(L) expression. The inhibition of this pathway is a new aspect of cytotoxic and modulatory potential of UCN-01.