A DP serological allospecificity was identified using 125I‐labeled preparations of HLA class II molecules isolated from cells of HLA homozygous typing cell lines, SLE (DRw6, DQw1, DPw3) and WT46 (DRw6, DQw1, DPw2), and depleted of DR molecules by absorption with an anti‐DR monoclonal antibody. The specificity, provisionally called WT, was carried by class II molecules possessing the characteristics of DP molecules on sodium dodecyl sulfate‐polyacrylamide gel electrophoresis, and detected primarily in DPw1, DPw3 and DPw5 cells and exceptionally in some DPw2 cells including WT46 and DPw4 cells on a large panel of DP‐pretyped B cell lines mostly derived from the 10th International Histocompatibility Workshop B cell reference panel. It was apparently allelic to another DP serological specificity BUT previously defined on DP molecules isolated from cells of DPw2 HLA deletion mutant cell line LCL 721.82. On the same cell panel, the BUT specificity was negative in all DPw1, DPw3 and DPw5 cells, and positive in all DPw2 and DPw4 cells and also in DPw2B and DPw4B cells except the cells typed WT. This DP association pattern was similar to that of the known allelic sequences, GGPM and DEAV, in DP ß F segment, one of the six variable segments in the second exon of DP ß gene. Thus, genomic DNA from 22 B cell lines pretyped for BUT and WT specificities were enzymatically amplified for the second exon of DP ß gene by the use of locus‐specific oligonucleotide primers and hybridized with 32P‐labeled oligonucleotide probes corresponding to the F segment sequence variations. This oligonucleotide typing showed perfect one‐to‐one correlation with the BUT and WT serological typing. The typing also revealed that WT46 cells, although typed DPw2, have the DEAV sequence common to DPw1, DPw3 and DPw5.