A sensitive and rapid method for the detection of monoclonal antibodies secreted by hybridomas is described. Mouse myeloma cells are fused with spleen cells from immunized mice and directly cloned in soft agarose containing selective medium; hybrid clones can be seen after a week. Nitro-cellulose filters that have been coated with a specific protein antigen, with antigen-coupled erythrocyte ghosts, or with other cells used as antigens are then placed on the agarose surface. After incubation to allow immunoadsorption of any secreted antibodies specific for the filter-bound antigen, the filter is removed and overlaid with a suspension of antigen-coupled erythrocytes that react with the adsorbed antibodies; after unbound erythrocytes are allowed to fall off the filter, red spots delineate the sites at which antibody-forming clones are present in the agarose. Alternatively, the filter may be treated with radiolabeled antigen followed by autoradiography. The reliability and sensitivity of the method are demonstrated with alpha-(1 leads to 3)-specific antidextran myeloma J558, and the method's applicability is established by detecting hybridomas with specificities for sheep erythrocytes and for alpha-(1 leads to 3) dextran.