The alpha-O-N-acetylgalactosaminyl-, alpha-3-D-galactosyl- and alpha-2-L-fucosyltransferases, the direct products of the respective A, B and H blood group genes have been identified in red cells, lymphocytes and platelets homogenates from secretor and non-secretor individuals of appropriate ABO group. The H enzyme was not detectable in red cell membranes from Bombay individuals. Since the uptake of A antigen from the plasma has been shown at least for red cells [10] and lymphocytes [25], the present results suggest that the ABH determinants of the blood cells may arise both from intrinsic and extrinsic origin, but the relative contribution of each mechanism is not known. In contrast, the Lewis enzyme was absent or inactive in all the investigated blood cell samples, which can be considered as an additional proof that the Lewis antigens of these cells are entirely derived from the plasma.