We have developed a modification of the solid-phase immunoassay for the detection of antibodies of specified isotype. This modified assay involves interaction of the antibody in solution with a haptenated antigen, followed by binding of the antigen-antibody complex to an anti-hapten immunoadsorbent. The antibody is subsequently detected with a suitably labelled antiglobulin reagent. The advantages of this test are: (1) The procedure is more convenient, especially in test protocols where multiple antigens are used. Because the antigens are in solution, computer-controlled dispensation of the antigens is possible. (2) The efficiency of antibody binding is increased, presumably because of the higher avidity of the antigen-antibody interaction in homogeneous solution compared to that in a heterogeneous system. Therefore, less antigen per test is required, and a solid phase with a lower capacity can be used.