The combined effects of x-irradiation and 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloro ethyl)-3-nitrosourea (ACNU) on multicellular glioblastoma A-7 spheroids were analyzed by means of cell survival and dose-response curves. The actual dose-response curve for small spheroids was almost identical to that estimated from the cell survival curve. It was strongly suggested that a small number of radiation-resistant cells, which were not detected in the cell survival curve, were present in

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... ge spheroids with central necrosis. The enhancing effect of ACNU was greater with large sphe roids than with monolayer cells or small spheroids. A possible explanation for this is that ACNU is higher effective against the few radiation-resistant cells that may be present in larger spheroids.