Identification of those patients with cancer who will or will not respond to a specific chemotherapy is important for making decisions regarding chemotherapy regimens as well as alternative management approaches. A laboratory test that could help to determine the sensitivity of an individual patient's tumor cells to specific chemotherapeutic agents would be valuable in choosing the optimal chemotherapy regimen for that patient with an expectation of increasing the response rate to the therapy. Several types of in vitro assays that measure tumor cell survival following exposure to cytotoxic agents have been evaluated for their ability to predict chemotherapy outcomes. As a group, these assays are referred to as drug resistance assays. In a resistance assay, the surviving tumor cells can be detected directly by their exclusion or metabolism of specific dyes. Alternatively, since some of tumor cells are proliferating, their survival can be detected by measurement of DNA synthesis by radiolabeled precursor incorporation or demonstration of clonogenic potential by growth into colonies in semi-solid culture medium. In several clinical studies, these assays were useful in detecting drug resistance and in predicting a poor prognosis for cancer patients. However, these resistance assays cannot detect sensitivity of an individual patient's tumor cells to a specific drug. Therefore, new methods determining drug-sensitivity of the tumor cells of an individual patient and, thus, capable of both predicting a positive treatment outcome and guiding chemotherapy, would be of significant value.