The modalities for the carrying out of a technique for the detection of HBs antigen, even when it is completely saturated by the relevant antibodies, and its results are set out. The technique includes the preliminary acidification of the test serum at pH 2.4, in such a way as to detach the antibody from the antigen, subsequently, the effect of the hearing (70 degrees C) for some minutes to destroy the antibody. On the contrary, at this temperature the immunological reactivity of the antigen remains satisfactory. Subsequently the neutralization is carried out with Veronal buffer, and the HBs Ag detection is performed by radioimmunoassay.