Intravenous injection to adult mice of Vi-antigen (200 microgram) induces in them the state of a short-term (10--12 days) unresponsiveness. This is due to the block of immunocompetent cells, not to masking the antibodies production by the excess of free antigen. Double washing to spleen cells before the test of local passive hemolysis in gel failed to reverse the block of the immune response; besides, there was no free antigen in the spleen that could inhibit the antibodies produced by the cells of the immune animal. The immune response block can be reversed by the injection of heterologous antiserum to Vi-antigen 18 to 24 hours before the Jerne's test. The restoration of immune response by means of the antiserum is prevented by the administration of 6-thioguanine after Vi-antigen (200 microgram). Thus, administration of a massive dose of Vi-antigen failes to block the proliferation and differentiation of the antigen-recognizing cells, but depresses the synthesis or secretion of antibodies.