Previous research has shown synergistic growth inhibition between UCN-01 and camptothecin (CPT) in tumor cells with mutant p53 versus tumor cells with wild-type p53. To determine the possible role of p53 in this drug combination, we tested the hypothesis that the synergistic growth inhibition is due to the absence of p53, and can result from the induction of DNA double-strand breaks (DSBs). Experiments were performed with the use of normal human mammary epithelial cells (HMEC); HMEC transfected with HPV16 E6 protein which inactivates p53 (HE6), or p53-mutant MDA-MB-231 tumor cells. CPT, UCN-01, or a 1: 1 combination of both, in either HMEC or HE6 cells did not induce DSBs. In contrast, simultaneous treatment of MDA-MB-231 cells with both UCN-01 and CPT induced significant levels of DSBs while treatment with either drug alone did not. While UCN-01 was surprisingly potent against HMEC, the growth inhibition was only additive between UCN-01 and CPT against these cells. HE6 cells were much less sensitive than HMEC to UCN-01 and slightly less sensitive to the combined treatment with UCN-01 and CPT. The drug combination was synergistic against HE6 cells, due to their lower sensitivity to UCN-01. Unlike what was observed previously in MDA-MB-231 cells, UCN-01 did not abrogate CPT-induced inhibition of DNA synthesis in either HMEC or HE6 cells. These data indicate that synergistic growth inhibition by UCN-01 and CPT against p53 mutant MDA-MB-231 tumor cells may be due to induction of DSBs however the loss of p53 function alone does not sensitize normal cells to the combination of both drugs.