Serological typing for the class II HLA-DR and -DQ specificities is important for clinical purposes such as renal transplantation, HLA-disease investigations, and HLA anthropological studies. In preparations of mononuclear cells from peripheral blood only B lymphocytes and monocytes (10 to 15% of the total cells) express the HLA-DR and -DQ molecules detected by serology. The HLA-DQ specificities are encoded by the DQA1 and the DQB1 genes, both of which are polymorphic and occur together in particular combinations. The cell surface HLA-DQ molecules are also dimeric a/p transmembrane glycoproteins, the antigenic characteristics of which are mostly conferred by the DQP chain, although the DQa chain is also variable. Antisera used for HLA-DR and -DQ typing are derived from individuals sensitized by nonself class II histocompatibility antigens. The majority of laboratories in the UK carrying out serological typing for class II antigens currently use immunomagnetic beads to separate out HLA-DR positive cells.