The ability to induce tolerance to uricase by the administration of native uricase, and uricase modified by the covalent attachment of monomethoxypolyethylene glycol (PEG) was examined. Uricase, and uricase with PEG attached to 35% (PEG-uricase 35%) and 70% (PEG-uricase 70%) of available amino groups were found to induce tolerance in mice not previously sensitized to uricase. There was a dampening of the IgG, IgE and IgM antibody response to uricase which persisted even after a second sensitizing dose of uricase was administered to these animals. PEG-uricases were found to have little or no immunogenicity when injected into mice and a reduced immunogenicity and antigenicity when tested in rabbits. Native uricase, however, was found to be immunogenic and antigenic in mice and rabbits. Mice sensitized to native uricase were injected with uricase, PEG-uricase 35% or 70% to induce tolerance. After a second sensitizing injection of uricase, circulating levels of IgE, IgG and IgM were measured. All three enzymes induced tolerance in the IgE class of antibody but there was no significant change in the hemagglutinating antibody levels of the mice. Mice injected with 1 mg of uricase died from anaphylaxis. PEG-uricase 35% was found to induce the most effective tolerance in both unsensitized and sensitized mice.