Among the DR specificities undefined by serology, DR-BON is peculiar because RFLP cannot distinguish it from the well-defined allele DR1 even if the two specificities are very different functionally. The occurrence of two DR-BON-like alleles in the same family, one associated to the DQw5 split of DQw1 and the other associated to the DQw7 split of DQw3, enabled us to compare the properties of these alleles. The RFLP analysis showed a typical DR1-like picture for both alleles when probed with DR beta, but for one of the alleles the DQ beta probe gave a DQw7 pattern. Primary mixed lymphocyte cultures showed weak to moderate stimulation between cells from individuals identical for one haplotype and differing for the DR-blank haplotypes, but by test with cloned reagents we were not able to define differences between the two DR-blank molecules. Two-dimensional gel electrophoresis and spot-test with a probe covering the third hypervariable region of the DRB1 gene showed no difference between the two alleles. We thus think that the two DR alleles are identical and that the stimulation observed in primary cultures probably is caused by incompatibility for DQ and DP or class I.