We have developed a new class of reagents (2) for the covalent attachment of polyethylene glycol to proteins. These reagents (2) are the monomethoxypolyethylene glycol esters of 4-fluoro-3-nitrobenzoic acid. The reaction of 2 with lysine epsilon-amino groups produces a chromophore which can be used to quantitate the polyethylene glycol to protein molar ratio. Bovine (Zn, Cu) superoxide dismutase was used as a model protein for conjugation with 2. When monomethoxypolyethylene glycol of average molecular weight 2105 was used, a conjugate was obtained with a polyethylene glycol to protein molar ratio of 8.88 retaining 100% of native enzymatic activity; monomethoxypolyethylene glycol of average molecular weight 5210 yielded a conjugate with a polyethylene glycol to protein molar ratio of 9.96 retaining 73% of native enzymatic activity.