Copper induced lysis of washed rat erythrocytes was stimulated by catecholamines, the order of effectiveness being: adrenaline greater than noradrenaline approximately equal to dopamine greater than L-DOPA. The degree of effectiveness is related to the rate of copper ion dependent oxidation of catecholamine, adrenaline being more rapidly oxidized than the other catecholamines investigated. Superoxide dismutase, catalase and different hydroxyl radical scavengers (mannitol, tris, formate and ethanol) markedly reduced the haemolytic effect of copper and catecholamine, suggesting the possibility that superoxide radicals and hydrogen peroxide, formed in the reaction system, cooperate in producing hydroxyl radicals, which are directly involved in the haemolytic action. The plasma proteins, ceruloplasmin, albumin and apotransferrin, also reduced the copper-catecholamine induced haemolysis. The protective action is probably not related to the copper binding ability of these proteins.