Horse liver alcohol dehydrogenase was carboxymethylated at the active site Cys-46 using 90% [1-13C]bromoacetate. The enriched carboxylate resonance was studied in the absence and presence of coenzymes and inhibitors. Previous ambiguities regarding the resonance in the NAD+ complex have now been resolved. However, it is shown that imidazole, an inhibitor introduced during the carboxymethylation, is not removed by the standard gel filtration step frequently employed and must thus bind much more tightly to the enzyme than suspected. Competition experiments involving imidazole and halide inhibitors show that the imidazole is preferentially bound when both are present in equimolar amounts. This suggests that the crystallographically identified anion binding site at the zinc of the carboxymethylated enzyme may require re-evaluation. The electron density at the zinc of the modified enzyme may be better explained as being due to unsuspected binding of imidazole.