Abstract A quantitative assay for soluble human HLA Class II antigenic material is described. The method is a double-determinant ELISA using 2 monoclonal antibodies which recognize different epitopes on the antigen. The first rat monoclonal antibody captures antigen onto the microplate where it is then recognized by a second mouse monoclonal antibody. Bound mouse immunoglobulin is then detected by an enzyme-linked rat polyclonal antibody. The assay has been used to measure soluble HLA Class II antigen in the sera from healthy individuals and patients with leukemia, and has shown raised levels in acute lymphoblastic but not in chronic lymphocytic leukaemia. Similar material has been identified in synovial fluid of patients with active rheumatoid arthritis. The assay is specific and simple to perform and should be applicable to probing the nature of the soluble material and its mechanism of release.