We devised a simple in vitro sensitivity test for lung cancer cells using Terasaki's microplate. We used the test to screen for sensitivity to various carcinostatic drugs and radiation, and to determine the optimum method of administration. This assay has been used in routine clinical examinations because about 40% of non-small cell carcinoma and 80% of small cell carcinoma of the lung can be subcultured. We describe here our patients who underwent treatment, various sensitivity tests and the preparation of an optimal course of treatment based upon the results of the sensitivity tests. Cells were placed in primary culture as previously described for short-term selective culture, and 2nd-3rd generation subcultured cells were transferred to individual wells of Terasaki's microplates for various sensitivity tests. After culture for 10 days, the effect was evaluated using 0.1% iodonitrotetrazolium (INT). This test permits a variety of sensitivity tests and various studies of clinical models of intensive treatment to be performed conveniently and reproducibly, because subcultured cancer cells are used. Another advantage is that these cells can be applied to basic investigations, including the preparation of monoclonal antibodies and chromosomes, DNA ploidy and oncogene studies.