NEW MATERIAL: An iron-succinylated polypeptide complex which is insoluble in water at a pH lower than about 6.5 and is soluble in water at a pH higher than about 6.5, or a salt of this complex and an alkaline (earth) metal or the salt thereof and an org. base.

USE: An oral therapeutic agent for iron-deficiency anemia having decreased side effects.

PROCESS: While the complex of, for example, succinylated protein and iron is kept at a specified pH of 6 to 10 in the presence of ≥1 kinds of proteases, the complex is decomposed by the enzymes until the desired iron-contg. succinylated polypeptide complex is formed under conditions of 0 to 50°C. The formed complex is recovered by dialystic filtration and freeze drying or impurities are removed by filtering the reaction mixture. The reaction mixture is then acidified to pH2 to 3.5 and, after the formed precipitate is redissolved at a neutral or basic pH, the product is recovered by the conventional process, by which the iron complex is obtd.