While the results of unrelated bone marrow transplantation are continually improving, a number of important issues remain: what are the histocompatibility requirements, what genes are involved, what mismatches are acceptable, and what are the best methods for determining donor-recipient match? In this study of material provided through the 4AOHW and the US NMDP, the match between 53 donor-recipient pairs was determined using several different markers within the MHC. The data showed that many apparently well-matched pairs have many mismatches, including mismatches for non-HLA genes (i.e., non-class-I or non-class-II) within the MHC. New methods matching for blocks of DNA around HLA-B and around HLA-DR/DQ are available that are sensitive and identify additional mismatches that are not apparent using conventional typing methods. The 4AOHW cells provide a valuable resource for the comparison and assessment of new matching techniques.