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.

Abstract: Muramyldipeptide active ester derivatives represented by the formula: ##STR1## wherein R.sub.1 represents a straight or branched chain fatty acid residue having 2 to 30 carbon atoms; R.sub.2 represents an active ester residue; and "Acyl" represents an acyl group having 2 to 6 carbon atoms. These compounds are applicable as haptens for the immunotherapy and exhibit antitumor activity.

Abstract: Muramylpeptide active ester derivatives represented by the formula: ##STR1## wherein R.sub.1 represents a hydrogen atom or a fatty acid residue having from 2 to 10 carbon atoms; --COR.sub.2 represents an active ester group; R.sub.3 represents a fatty acid residue having from 1 to 10 carbon atoms; X represents an alanine, N-methylalanine or valine residue; "Acyl" represents a fatty acid residue having from 2 to 6 carbon atoms; and n represents an integer of from 1 to 6. These compounds are applicable as haptens for the immunotherapy and exhibit antitumor activity.