The haemolytic activity of iron-dextran complex is found to be a function of time, temperature, pH, and concentration. The lytic action is enhanced by small amounts of added ferrous sulphate. The lytic action is inhibited by chelating agents such as citrate and sequestrene salts, which bind ionic iron, but not by ferric citrate or ferric sequestrene which do not bind iron. The ionised iron content of iron-dextran is deduced. The lytic activity of iron-dextran is also inhibited by iron-dextrin and by an iron-sorbitol-citric acid preparation. It is suggested that the iron-sorbitol-citrate molecular complex contains free chelating groups for iron. The significance of these findings for iron-carbohydrate toxicity and metabolism is briefly discussed.