The inventive method for the production of crystalline (6RS)-N(5)-formyl-5,6,7,8-tetrahydrofolic acid or amorphic (6S)-N(5)-formyl-5,6,7,8-tetrahydrofolic acid is characterized in that an aqueous solution of (6RS)- or (6S)-calcium folinate, which has a temperature of 40° C. to 50° C., and an aqueous solution of hydrochloric acid or acetic acid are added to stirred water having a temperature of 2° C. to 12° C., such that the temperature is kept at 2° C. to 12° C. in the mixture thus obtained when the two above-mentioned solutions are added and the pH value is kept at 2,5 to 3,5, the solid thus arising is isolated by means of filtration or centrifugation, the solid is initially washed with cold water and then with an aqueous organic solvent, and the washed solid, i.e. crystalline (6RS)-N(5)-formyl-5,6,7,8-tetrahydrofolic acid or amorphous (6S)-N(5)-formyl-5,6,7,8-tetrahydrofolic acid is dried at reduced pressure and obtained.
