The technical aspects and preliminary results of an in vitro radioassay for tumor chemosensitivity are described. A representative series of gynecologic cancers has been studied. The procedure monitors inhibition of uptake of protein and nucleic acid precursors and requires only short incubation periods and small numbers of cells. Our initial data using tritiated thymidine and a variety of common chemotherapeutic drugs indicate that significant short-term (2 h) isotope uptake takes place and inhibition by several drugs can consistently be demonstrated. However, it is also obvious that the principal effects observed relates in most cases to the specific mode of action of individual drugs on precursor pathways, rather than intrinsic cytotoxicity on each tumor strain. The limitations of the method and a partial solution to the major problems in data interpretation are briefly discussed