The D region of the HLA system is very complex. Three loci, HLA-DR, DQ and DP, are now very well established. However, the question of the independence of the first known locus of this region remains elusive--it is not yet known if HLA-D antigens are really independent or if they represent some epitopes on the molecules of HLA-DR/DQ antigens. The HLA-DP antigens represent an independent series in the HLA system recognized by the PLT technique. On the other hand, the HLA-PL antigens also detected by PLT correlate with both HLA-D and HLA-DR antigens, and it seems that they do not constitute an independent group of HLA antigens.