HLA-DP genotyping was performed using dot-blot analysis with synthetic oligonucleotide probes. Fourteen probes were designed based on the known sequence variations in the polymorphic segments of the DP beta second exon. Each probe was tested against genomic DNA amplified by the polymerase chain reaction, using DP beta-specific primers. A total of 45 HLA homozygous B-cell lines, selected from the Tenth International Histocompatability Workshop and pretyped for the known DP omega specificities, were analyzed. Different hybridization patterns were found for each DP omega specificity. The oligonucleotide hybridization performed on DP omega-negative B-cell lines gave a pattern distinct from those of known DP omega specificities, indicating the presence of novel DP allelic sequences. The use of sequence-specific oligonucleotides combined with DNA amplification allows a simple and reliable genotyping of DP antigens.