Multicellular [Chinese hamster V79 lung] spheroids grown in vitro provide a convenient model of nodular tumors for experimental tumor therapy studies. Adriamycin inactivated cells grown as spheroids less efficiently than single cells, presumably due to enhanced cellular resistance analogous to the increased radioresistance observed when these cells are grown in close contact. Spheroid growth was retarded by a minimally toxic (0.005 .mu.g/ml) chronic level of adriamycin. Irradiation and exposure to that drug concentration were not synergistic. Larger adriamycin concentrations (0.5 .mu.g/ml) present during radiation exposure produced marked radiosensitization, presumably due to the drug inhibiting cellular 02 consumption and permitting reoxygenation of the previously hypoxic spheroid cells.