A new guanosine analogue has been synthesized, 5'-p-fluorosulfonylbenzoyl guanosine, which has an electrophilic moiety capable of reacting covalently with several classes of amino acid side chains found in proteins. This compound reacts with bovine liver glutamate dehydrogenase to desensitize it irreversibly to inhibition by GTP, without affecting its intrinsic catalytic activity. The specific addition of GTP or GTP and TPNH to the reaction mixture prevents the loss of sensitivity to GTP inhibition. The corporation of approximately 1 mol of 5'-p-sulfonylbenzoyl guanosine/enzyme subunit is associated with the decreased responsiveness of the enzyme to regulation by GTP. It is proposed that 5'-p-fluorosulfonylbenzoyl guanosine may be reacting within the allosteric GTP site of glutamate dehydrogenase and that this compound may have general applicability in the affinity labeling of regulatory and catalytic sites of proteins which normally bind guanosine nucleotides.