HLA-D/DR (Ia) glycoproteins were identified in human breast carcinoma and normal mammary gland cells by means of an anti-Ia monoclonal antibody. Two techniques were used: (1) immunoperoxidase staining performed on histological sections and (2) Ia glycoproteins were isolated as follows: firstly by radioactive labelling of isolated cells, then by filtration on Sephadex G25, followed by Lens culinaris chromatography, and immune complex formation and then elution on protein A-Sepharose. Lastly, the immune complex was studied by chromatofocusing. Both techniques revealed that Ia expression was found in carcinoma cells, but not in normal cells.