Recipients (4076) and donors (3325) of kidney transplants performed at 110 transplant centers were typed for HLA-DRB by the DNA RFLP method. The discrepancy rate of replicate samples distributed among 8 participating laboratories was a low 2.6%. The discrepancy rate between RFLP-DRB and serological HLA-DR typings was 25.0% for organ donors and 27.6% for kidney recipients. Discrepancy rates at the different transplant centers ranged from 9.7% to 86.7%. The discrepancies consisted of antigens being incorrectly interpreted by serology (16.8%), and of serological "blanks" turning out to be definable alleles by the DNA method (10.8%). The alleles that were mainly affected by discrepancies were DR1, DR8, DR10, DR12, DR13, DR14, DR16, DR17.2, and DR18.