Abstract: A process for producing an antibody having a high specificity to a first antigen and a low cross-reactivity with at least one second antigen, said first antigen comprising a desired antigenic determinant and said second antigen comprising at least one antigenic determinant which is structurally related to said desired antigenic determinant of said first antigen, which process comprises treating a mammal with a copolymer of D-glutamic acid and D-lysine coupled with said second antigen whereby to induce a substantially effective immunological tolerance to said second antigen and then immunizing said mammal with said first antigen. This process results in a higher productivity of a mammal cell capable of producing said desired antibody by culturing said mammal cell, for example, by forming a hybridoma with a suitable tumor cell and implanting the hybridoma into another mammal. The antibody or antiserum of this invention may with advantage be used for immunoassay such as radioimmunoassay.