Chemosensitivity is affected by molecular biological factors, including factors related to the induction of apoptosis and the activity of proliferation. We analyzed immunohistochemically the expression of p53, Bcl-2, and Ki-67 in various types of cancers and assessed the correlation between this expression and chemosensitivity. Moreover, we investigated whether the expression of these factors could be a useful predictor for the clinical response to chemotherapy. Study subjects comprised 63 preoperative patients with untreated malignant tumors (9 with esophageal cancer, 12 with stomach cancer, 12 with colon cancer, 16 with liver cancer, and 14 with breast cancer). Immunohistochemical staining (the labeled streptavidin biotin technique: LSAB method) was used to assess expression of p53 protein, Bcl-2 protein, and Ki-67. A chemosensitivity test was carried out with the histoculture drug response assay method using four drugs: mitomycin C, 5-fluorouracil, doxorubicin hydrochloride (ADM), and cisplatin (CDDP). Immunohistochemical studies for p53 were found to be useful for predicting chemosensitivity.