Human major histocompatibility complex class II antigens are polymorphic cell surface heterodimers encoded by distinct a and p-chain genes. They play a key role in the control of the immune response to pathogens, in transplantation immunology, and in the susceptibility to autoimmune diseases. Histocompatibility testing allows the matching of patients and donors, and the clinical outcome of organ and bone marrow transplantation depends to a large extent on the accuracy of HLA matching. This chapter presents a detailed protocol for HLA-DR generic oligotyping and for the analysis of 53 DRB1,3 DRB5,3 DRB4,17 DQB1, and 22 DPB1 alleles. Recently, on the bases of the reverse dot blot technology40 and of the sandwich hybridization, this DR generic typing procedure has been further simplified by using oligonucleotide-coated microtiter plates for a 1 h-hybridization step at 37°C, followed by automated washing and colorimetric reading of the plates.