DNA typing of HLA class II alleles of the DRB1/3/4/5 and DQB1 loci using sequence-specific oligonucleotide probes and polymerase chain reaction amplified DNA has been used for the large-scale typing of donors for the National Marrow Donor Program unrelated donor registry. The results of quality control analysis for the second year of the project (10/1/939/30/94) show the typing continues to be highly accurate, specific, and reliable. The average percent of correctly classified HLA oligotypes (groups of alleles defined by a hybridization pattern with a panel of sequence-specific oligonucleotide probes) based on 9,244 DRB1 and 7,244 DQB1 assignments was 99.8% (range 99.4%100.0%) for DRB1/DRB3/DRB4/DRB5 and 99.8% (range 99.6%100.0%) for DQB1. This level of accuracy is particularly remarkable because the 4,636 DRB quality control samples were tested blindly and could not be distinguished from 57,580 donor samples tested at the same time by the laboratories.