The mechanisms of unusually weak A and B blood group expressions have not been well understood. Since the human blood group A and B substances are produced by the action of blood group GalNAc transferase and Gal transferase, respectively, the mechanism may be elucidated by examining the properties of the blood group transferases and membranes of the subjects with the abnormality. We examined a case associated with very weak B activity in red blood cells, an absence of the B agglutinin in serum, and an existence of the H and B substances in saliva, i.e., a case commonly classified as Bm. More than 85% of H sites remained unglycosylated in the subject's red cell membranes. The blood group Gal transferase activity in the subject's plasma and red cell membranes was about 50% of that of normal. The pH-activity profile and the Michaelis constants for UDP-Gal and 2'-fucosyllactose of the subject's enzyme were distinctively different from that of normal enzyme. These findings led us to conclude that the weak B activity in the present Bm case was due to a direct mutation in B gene resulting in formation of variant B enzyme with low affinity to UDP-Gal and insufficient galactosylation of H sites in the subject.