Insulin, stated to be pure and to have a slower and more prolonged hypoglycemic effect than ordinary insulin, is obtained by adding to a solution of insulin a buffer, such as a salt of a strong base and a weak acid, in an amount of 1/5 to 1/40 molar, adding a water soluble salt of cadmium, nickel, calcium or cobalt with hydrochloric, acetic or tartaric acid, cooling the solution and recovering the precipitate. After the addition of the cadmium, nickel, calcium or cobalt salt an organic solvent, e.g. acetone to the amount of 12 per cent, is added, the solution heated to between 20 and 65 DEG C. cooled and separated from any flocculent precipitate, the pH is adjusted to between 5,6 and 6,4 and the solution is cooled to near freezing point, when a crystalline precipitate is formed which is the insulin product desired. In the instance of nickel chloride as the precipitating agent one part thereof to 10,000 of the mixture may be added. For use the insulin prepared as above is introduced into a buffer solution, e.g. of sodium phosphate, and brought to a pH approximately that of the body tissue.