An iron preparation suitable for intramuscular injection should conform to the following criteria: (a) it should be rapidly absorbed from the muscle and the total iron content should be available for haemoglobin synthesis, (b) in order to avoid pain, the pH and tonicity should approximate to those of normal tissue fluids, (c) it should be stable in the presence of protein and electrolytes, (d) it should possess a low toxicity, and be stable on storage, (e) in order to keep the injection volume down to a minimum, the solution should contain at least 5% w/v of iron.Previous workers have described the subcutaneous or intramuscular administration of compounds containing either ionic iron (Brownlee, 1942; Goldberg and Hutchison, 1953) colloidal ferric hydroxide, or saccharated oxide of iron (Slack, 1949), but preparations which contained sufficient iron to be useful in the treatment of hypochromic anaemia were found to be either toxic or painful.Initially, we investigated two series of iron compounds. In the first series, the iron was present as a co-ordination compound, e.g., ferrous calcium ethylenediaminetetraacetate, and other ferrous or ferric alkali, alkaline earth, or organic base salts of ethylenediaminetetraacetic acid. In the second series, the preparations were based on saccharated oxide of iron, but other sugars were used in place of sucrose. On investigation in animals, compounds of the first series caused systemic haemorrhage and were rejected; none of the preparations in the second series showed satisfactory absorption from muscle, and some were, in addition, very toxic.Iron is stored in the body as a macromolecular protein complex, ferritin, and is also transported in combination with the protein, al-globulin. It was decided, therefore, to investigate the substitution of the protein moiety by other macromolecules; a third series was therefore investigated in which the 2C