In this study, we determined whether p53 expression affected the sensitivity of non-small cell lung cancer (NSCLC) and colon cancer cells to bleomycin (BLM). Two human NSCLC cell lines (A549 expressing wild-type p53 and p53-null H1299) and two colon cancer cell lines (HCT116 p53+/+ and its p53 deficient subline HCT116 p53-/-) were subjected to treatment with BLM. Cells were treated with various concentrations of BLM, and cellular viability was assessed by formazan assay. To investigate the role of p53 in BLM sensitivity, we transduced cells with adenovirus with wild-type p53, dominant-negative p53, and GFP control, and analyzed the effect on cellular viability. Cells expressing wild-type p53 were more sensitive to BLM than p53-null cells in both NSCLC and colon cancer cells. Sensitivity to BLM of the cells with wild-type p53 was reduced by overexpression of dominant-negative p53, while BLM sensitivity of p53-null cells was increased by wild-type p53 in both NSCLC cells and colon cancer cells. In conclusion, our data show that p53 sensitizes all four cancer cells lines tested to BLM toxicity and overexpression of p53 confers sensitivity to the cytotoxic activity of the anticancer agent in p53-null cells.