The HLA-DR molecule is a polymorphic membrane glycoprotein and one of the key molecules causing allograft rejection and graft-versus-host disease in organ transplantation. Serologic typing using DR specific alloantisera has long been used, but several problems have limited its application. The purpose of this study was to establish an efficient reverse-SSO typing system that detects DRB1 and DRB3/B4/B5 alleles on a single membrane. A DR typing membrane was prepared by immobilizing 21 dT-tailed sequence specific oligonucleotides (SSOs) on a nylon membrane and was used in a hybridization assay with digoxigenin-labeled PCR-amplified target DNA. The positive signals were detected on X-ray film using chemiluminescence. A comparison study with serology using DNAs from 105 unrelated individuals demonstrated that the reverse-SSO typing system was superior to serologic typing in terms of accuracy (100% vs 90.5%), simplicity, range of application, rapidity, and cost of the test. These data indicate that the reverse-SSO typing system can replace serology as a routine DR test, and will be useful in time-restricted solid organ transplantation and in selection of an unrelated marrow donor prior to bone marrow transplantation.