From the results of our studies presented thus far in details, we may tabulate the essential facts and draw our conclusion. With respect to A•B•O blood types, the following points are to be reemphasized: 1) The existence of A- and B-type substances as well as O-type substance is proven clearly at about the second month of foetal life. 2) In A- and B-type substances, during the foetal periods, the lower partial antigens are developed far more than the higher partial antigens. 3) In O-type substance, there exists no recognizable difference in development among OI, OII and OIII partial antigens. 4) During the first half of the foetal period, the development of O-substance is most striking, that of B-substance is less and that of A-substance is least. 5) A-, B- and O-substances all continue to develop from the foetal period, though their velocities may vary, and reach 50-70% of the adult value at parturition. Their development continues thereafter and the values come close to that of the adult at 5-7 years of age.