Two paternity cases involving black persons are reported in which possible paternal exclusions existed if the A2B red cell phenotype represented the genotype A2B. ABH transferase assays were performed to determine if the A2B phenotypes arose from the genotype A1B or A2B. These assays confirmed that all group A members carried the A1gene and not the A2gene. Therefore, the alleged father was not excluded in either case. The B transferase showed increased activity in both AB individuals, possibly accounting for the observed A2B red cell phenotypes. Therefore, in the presence of aBgene, accurate assignment of A subtypes using standard serological methods may not be possible.