Soluble cross-linked polymers of hog-liver uricase and an excess of either rabbit or dog albumin were injected repeatedly into rabbits in order to determine their antigenicity and immunogenicity. Whereas the enzyme in its free form induces antibody production, homologous albumin polymerised with the enzyme renders the complex non-immunogenic and non-antigenic. Dog-albumin/uricase polymers injected into rabbits induce antibody formation against the dog albumin but not against the uricase. The results suggest an important advantage of these soluble enzyme polymers in enzyme-replacement therapy.