An in vitro assay was developed to measure the chemotherapeutic drug susceptibility of cells from human tumors. The assay utilized live cells, freshly isolated from tumor tissue, which were incubated for a short period in vitro. The drug-induced inhibition of incorporation of radiolabeled precursor into DNA, RNA, and protein was measured. The assay is sensitive to concentrations of chemotherapeutic drugs in the therapeutic range and is reproducible when tested with replicates of the same tumor cell population.