A technique is described for enriching, from human blood, cells bearing HLA-DR antigens. The method depends on the use of monoclonal mouse antibody which reacts with HLA-DR structures. Cells to which this antibody has bound can be separated after rosetting with bovine erythrocytes coated with anti-mouse immunoglobulin. The cells thus enriched may be used for HLA-DR typing by standard cytotoxicity methods with allogeneic sera.