During more than 9,000 Gm(a)-type determinations with two Ragg anti-Gm(a) sera, ‘1604’ and ‘Esra’ 4 Gm(a+) sera failed to inhibit ‘1604’. ‘1604’ reacted with coats capable of Gm(a), (x), and (z) typing, not with coats for Gm(f), (b), and (g) typing. Absorption of’1604’ with erythrocytes coated with anti-D sera for Gm (f), (b), and (g) typing did not remove the agglutinating capability against coats for Gm(a) typing. Red cells coated with the anti-D employed, ‘07’, failed to react with anti-Gm(f), (b), and (g) reagents. A family investigation, including Gm(n), (z), (a), (x), (g), (f), (b1), and (b3) typing, showed the antigen detected by ‘1604’ to be inherited in the same way as the (other) Gm factors. It is concluded that ‘1604’ demonstrates an inheritable Gm factor, present in almost all Gm(a+) sera, carried by the same molecules as Gm(a), (x), and (z), and not identical with any known Gm factor. Considering the close association between Gm(a) and this new factor, and the obvious resemblance with the relations between the corresponding Gm(b) factors in Caucasians, the symbols Gm(a°) and GmΔa1) are provisionally used for Gm(a) proper and