Exposure of Ehrlich ascites tumor cells to the polyanion dextran sulfate altered the permeability of the plasma membrane resulting in decreased glycolysis and respiration, enhanced permeability to Rb+ and ATP and increased staining by erythrosin B. The inhibition of respiration was reversed by the addition of inorganic phosphate, whereas restoration of glycolysis required inorganic phosphate and AMP. The membrane lesion was repaired by injection of dextran sulfate treated cells into mice or in vitro exposure to a heat stable extract from ascites fluid. Repaired cells were no longer stained by erythrosin B and glcolyzed without added AMP. The time course of repair was separable into a rapid initial repair of the membrane lesion followed by a slower recovery of the normal intracellular ionic composition. Both phases were insensitive to inhibitors of protein synthesis. The repair activity of ascites fluid was heat stable and was composed of dialyzable and nondialyzable factors. Incubation of dextran sulfate treated cells with ascites fluid restored Rb+ uptake and abolished ATP-stimulated Ca2+ uptake.