Escherichia coli tryptophanase was modified with 2,4-bis(0-methoxypolyethylene glycol)-6-chloro-s-triazine (activated PEG(2), MW 5,000 X 2). The modified tryptophanase,in which approximately 43% of the total 120 amino groups and 38% of the total 16 sulfhydryl groups in the molecule were coupled, completely lost the immunoreactivity towards anti-tryptophanase serum from rabbit. Approximately 10 % of the enzymic activity was retained. The modified enzyme showed the same physicochemical properties as the native enzyme: K(m) value for L-tryptophan (0.3 mmol/1), optimum pH (8.0) and optimum temperature (50 °C). The modified enzyme was more resistant than the native counterpart against proteolytic digestion with trypsin.