The cytotoxicity of two antineoplastic drugs, cisplatin and CCNU, was evaluated in Chinese hamster V79 multicell spheroids using the drugs as single agents or combinations. Cells obtained from different depths within spheroids 550-750 microns in diameter showed different sensitivities to the two agents; the external cells of the spheroids were more sensitive than the internal cells to cisplatin, whereas the internal cells were most effectively killed by CCNU. Combining the two agents produced the expected 'complementary' activity, and in addition, synergism was observed between the drugs at exposure levels practical for clinical use. For the combination treatments, both the net pattern of cell killing through the spheroid and the degree of interaction between the agents (quantified using the combination index method) were a function of the dose ratio of the two drugs, and of overall treatment intensity. BCNU produced patterns of cell killing similar to CCNU, but showed little interaction with cisplatin. Our results suggest significant clinical potential in using CCNU with cisplatin, particularly since CCNU-cisplatin combinations were synergistic even in the cell subpopulations most resistant to each drug as a single agent.