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Cell survival fraction for HEp 2 cells, a human epidermoid carcinoma of the larynx, was determined by exposing the cells to 4 MV linear acceleration radiation. A D0 value of 140 +/- 20 rad was measured at a dose rate of 300 rad/min. The cells were also treated concurrently with partially cytotoxic drug doses of Actinomycin D (ACD) and Adriamycin (ADRM) separately and in combination with radiation in an effort to characterize possible enhanced effects for the three component reaction species. Enhancement ratios of 1.05 +/- 0.15 for the drug-drug interaction, 1.20 +/- 0.19 (ADRM-radiation) and 1.23 +/- 0.21 (ACD-radiation) for the radiation-drug combinations and 1.28 +/- 0.17 for the drug-drug-radiation combination were measured. These data suggest that an additive response occurs with each of the drugs and radiation taken individually but little further significant enhancement is seen when all reaction components are used in combination.