We have investigated whether variability in the apoptotic pathway may account for the differential susceptibility to apoptosis-induction by 9-nitrocamptothecin (9-NC) in cell subpopulations derived from the human ovarian cancer cell line, SKOV-3. Quantitative differences in the apoptotic fractions of cells were assessed by flow cytometry, whereas major regulatory and executing components of the apoptotic machinery were investigated by Western blot analysis using specific antibodies. The results indicate that indeed the apoptotic pathway was activated by 9-NC in some, but not all, cells of the SKOV-3 cell line, suggesting that 9-NC alone may partially be effective for treatment of patients with ovarian cancer.