The mechanisms of unusually weak blood group (A and B) expressions are not yet well understood. We examined properties of blood group galactosyltransferase (B-enzyme) and characteristics of red cell membrane components obtained from family members with A2Bm character. B- enzyme activity of the A1Bm plasma is in normal range, and kinetic properties (i.e., Km for UDP-Gal, Km for 2′-fucosyllactose, and pH optima) of B-enzyme from the A1Bm subjects are identical to that of normal B-enzyme. When A1Bm red cell were incubated with UDP-Gal and B- enzyme, the cells became strongly agglutinable with anti-B. When A1Bm membranes were incubated with B-enzyme or A-enzyme (i.e., blood group N- acetylgalactosaminyltransferase) and the appropriately labeled nucleotide sugar (UDP-Gal3H for B-enzyme and UDP-GalNAc3H for A- enzyme), significant incorporation of the sugar was observed. The amounts of the sugar incorporated into A1Bm membranes were about 40%- 50% of that incorporated into O membranes at saturation, indicating that about one-half of H-sites remained unglycosylated in A1Bm red cells. Examination of radioactive components by isoelectric focussing revealed that the labeled components of A1Bm membranes were distinctively different from that of O membranes. Therefore, one can conclude that the weak B expression is not due to direct mutation of ABO locus, but due to a secondary consequence of genetic abnormality of a membrane component (or components) associated with blood group substances.