Thorough examination of ABO blood type in cynomolgus monkeys is an essential experimental step to prevent humoral rejection during transplantation research. In the present study, we evaluated current methods of ABO blood-antigen typing in cynomolgus monkeys by comparing the outcomes obtained by reverse hemagglutination, single-nucleotide polymorphism (SNP) analysis, and buccal mucosal immunohistochemistry. Among 21 animals, 5 were type A regardless of the method. However, of 8 serologically type B animals, 3 had a heterozygous type AB SNP profile, among which 2 failed to express A antigen, as shown by immunohistochemical analysis. Among 8 serologically type AB animals, 2 appeared to be type A by SNP analysis and immunohistochemistry. None of the methods identified any type O subjects. We conclude that the expression of ABO blood-group antigens is regulated by an incompletely understood process and that using both SNP and immunohistochemistry might minimize the risk of incorrect results obtained from the conventional hemagglutination assay.