In a method for preparing a monoclonal hybridoma which secretes antibodies against an immunogenic material, comprising sensitizing an appropriate source with an immunizing amount of a preparation of the immunogenic material, obtaining from the source lymphocytes having immune activity against the immunogenic material, and fusing the lymphocytes with an appropriate cell line to thereby form mixed hybridomas, the improvement wherein: the immunogenic material is present in the immunizing preparation in an amount not larger than 10% by weight, and wherein, after formation of the mixed hydridomas but prior to cloning, the method comprises the steps of: raising ascites fluid with the mixed hydridomas, then detecting immune specificity against the immunogenic material in the ascites raised fluid; and thereafter cloning the ascites raised cells which show immune specificity against the material.