The effect of neurotropin (a drug with antioxidant action) on the deformability and electrophoretic mobility (EM) of rabbit erythrocytes was investigated and compared to that of endotoxin from E. Coli (0111:B4) using an electrooptical technique. Endotoxin decreased the erythrocyte deformability in vitro. Upon incubation with neurotropin the endotoxin effect was reversed, i.e. the deformability of the erythrocytes increased. The electrophoretic mobility of red blood cells exposed to the simultaneous action of endotoxin and neurotropin was substantially closer to that of the control cells, as compared to the one of erythrocytes incubated with endotoxin alone. Neurotropin (1 mg.kg-1 given 1 hour after endotoxin administration) decreased the mortality of the animals, injected with endotoxin (2 mg.kg-1) namely by in the first day 63.6% versus 9.1%.