A 68-year-old man with an ABO discrepancy discovered during a routine check up and 3 members of his family were referred for ABO analysis. A novel allele (A(w)14; 699 C>A, H233Q) was found by sequence analysis of exons 6&7 and in those with the A(w)14 allele, complete ABO analysis was performed. The propositus and his son had an A(w) phenotype with the A(w)14/O01 genotype. His daughter had an A(w)14/B101 genotype with a normal B phenotype. Three-dimensional molecular modelling was used to predict the effect of the H233Q amino acid change on the resulting A glycosyltransferase. The A(w)14 produced a weak A phenotype, and its ability to produce A antigens was further reduced when it was co-inherited with a B101 enzyme.