Abstract: The presence of alloantigen allelic products or antibodies to alloantigen allelic products is determined by employing a support to which alloantigen allelic products are bound, and combining a specimen which contains either the alloantigen allelic products or antibodies to alloantigen allelic products with a reagent comprising antibodies to the alloantigen allelic product(s) of interest. By combining the specimen, reagent and support, and determining the amount of antibody which binds to the support as compared to a negative control, the presence of the alloantigen antigen or anti-alloantigen is related to a reduced amount of antibody binding to the support. The amount of antibody may be determined by any convenient assay.