Numerous procedures have been described which test the chemosensitivity of tumor cell lines. A major disadvantage of most of these assays is that practical limitations prevent the testing of more than a few variables. We have adapted a rapid and efficient colorimetric assay for testing the chemosensitivity of human lung tumor cells. In this assay, a tetrazolium salt (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide, MTT) is converted to a colored formazan product by enzymes active only in living cells. The MTT assay may be carried out entirely in 96-well microtiter plates, so that large experiments examining a number of variables can be readily performed. Thus, drug concentration, time of exposure to drug, length of assay, and cell density can be varied and tested. Moreover, the simplicity of this assay allows simultaneous testing of multiple drugs on multiple cell lines. Finally, the MTT assay is useful for monitoring the development of multidrug-resistant cells in culture.