To provide a method for establishing compatibility of two blood types on the basis of cross matching tests of a small number of blood type genotypes between a recipient and a future donor.

Blood type genotypes are mapped to corresponding phenotypes by a manifestation condition related to a set based on haplotypes in order to determine compatibility, and the compatibility is established by establishing compatibility of blood types constructed as a combination of composition phenotypes. It is preferable that bit symbol strings coincide with each other and an algorithm arithmetic expression is used. When ambiguity in mapping of genotypes to haplotypes exists, the ambiguity can be reduced on the basis of occurrence frequency of the haplotypes in sample population or solved by gametic phasing. Such reduction or resolution of ambiguity is especially desired when mismatching of antigens manifested by a haplotype of compositions has larger clinical significance.