Antibodies were covalently conjugated with poly(ethylene glycol) (PEG) and the properties of the PEGylated antibodies in organic media were investigated. Two types of monoclonal antibody were used in this study. One was a monoclonal antibody (abzyme) that was prepared against a hapten mimicking a transition state of hydrolysis. Another was a monoclonal antibody against estrogen, which is not soluble in water. By electrophoresis and mass spectral analysis, the covalent conjugation with PEG chains was confirmed. The PEGylated antibodies bound to antigens and the PEGylated abzyme catalyzed a hydrolysis reaction in an aqueous solution. The PEGylated antibodies were soluble in dichloromethane and acetone and interacted with antigen either in dichloromethane or in acetone. In conclusion, PEGylated antibodies can be employed as analytical tools for water-insoluble analytes. (C) 2011, The Society for Biotechnology, Japan. All rights reserved.