Recipient mismatching for the minor histocompatibility antigen HA-1 has been associated with acute graft-versus-host disease after allogeneic marrow transplantation. Two polymorphic nucleotides near an exon-intron junction of the gene encoding this minor histocompatibility antigen have been identified. In this study, we determined the genomic DNA sequence of the intron downstream from this polymorphic exon. Based on this sequence, primers were designed to amplify the genomic HA-1 gene sequence, and analysis of restriction fragment length polymorphisms was used to assign the HA-1 genotypes of 160 unrelated probands and a paired sibling for each proband. Among probands, the HA-1H allele frequency was 0.441, and the HA-1R allele frequency was 0.559. The distribution of HA-1 genotypes showed close fit with Hardy-Weinberg equilibrium. Likewise, the number of sibling pairs with disparity for HA-1 alleles showed close fit with predictions based on Hardy-Weinberg equilibrium. These results provide a simple and well validated method for future studies correlating HA-1 disparity with clinical outcome after allogeneic marrow transplantation.