A subline of L5178Y cells has been established in vitro that exhibits a fiftyfold order of resistance to 5-fluorouracil (FUra) as compared to that of the parent line. The cytotoxic effects of 24-hour exposures to 23 antitumor drugs and to radiation were compared in the two cell lines. Four patterns of response were identified: 1) Only two drugs, mitomycin C and adriamycin, proved significantly more cytotoxic to FUra-resistant cells. 2) Four other drugs--anguidine, 4'-(9-acridinylamino)-methanesulfon-m-anisidide, melphalan, and quelamycin--showed marginal superiority against resistant cells. 3) X-radiation and the majority of drugs tested--including 5-azacytidine, 1,3-bis(2-chloroethyl)-1-nitrosourea, cisplatin, bleomycin, dibromodulcitol, razoxane, hydroxyurea, methotrexate, teniposide, etoposide, and three experimental agents, metoprine, spirogermanium HCl, and ellipticinum--proved equally cytotoxic to both cell lines. 4) Cross-resistance with FUra was exhibited with vincristine, vindesine, pyrazofurin, and indicine-N-oxide. This experimental system provides a simple method of testing agents for activity against FUra-resistant cells before phase 1 clinical studies.