Despite the importance of the guinea pig as a model test animal for studying the humoral immune response in comparison to the development of cellular immunity after BCG or tuberculosis infection, no precise and sensitive method for the determination of such antibodies is available. We developed an indirect solid-phase radioimmunoassay on microtiter plates for this purpose. All assay conditions have been optimized such as selection of suitable plastic material, use of carrier substances, concentrations and amount of the antigen as well as the serum and second antibody, buffer, incubation times and all steps of the procedures for coating, washing, and measuring. The assay gives a linear relationship between antibody concentration in the serum to be tested and binding of the radiolabelled second antibody. A comparison with four conventional antibody titration methods reveals a 128- to 512-fold higher sensitivity of the RIA. Favorable aspects of this RIA as opposed to the methods hitherto used are discussed.