For the successful administration of exogenous proteins and enzymes for therapeutic purposes, abrogation or modifications of antigenicity and immunogenicity of the protein must be achieved without substantial loss of the biological activity necessary for therapeutic purposes. In order to try this, a model experiment was carried out in the present study. Escherichia coli L-asparaginase, whose chemical properties have been well defined and which is used for the treatment of acute lymphocytic leukemia and malignant lymphomas, was modified by copolymerization with isologous albumin, and alterations of its immunogenicity were examined in mice. The results demonstrated that the immunogenicity of the modified L-asparaginase was remarkably reduced and satisfactory enzyme activity was preserved.