The purified uricase (urate: oxygen oxidoreductase, EC 1.7.3.3) from Candida utilis was modified to varying degrees with monomethoxypolyethylene glycol (PEG) of different molecular weights using cyanuric chloride as the coupling reagent. The PEG-uricase conjugates were examined on their immunological properties by means of ring test and passive cutaneous anaphylaxis (PCA). As increasing amounts of PEG were attached to uricase, it showed decreasing ability to elicit antibody production in rabbits. When sufficient polymers were attached, the modified uricase was devoid of the capacity to combine in vivo and in vitro with antibodies from guinea pigs injected with the unmodified uricase, however, were still able to react with antibodies to PEG-uricase conjugate. Antibodies against PEG-uricase conjugates also reacted with PEG modified superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1). These results indicate that the coupling of PEG to uricase resulted in the loss of original antigenicity and immunogenicity, but in the appearance of new antigenicity and immunogenicity which never showed any cross-reactions against the native uricase.