Addition of 1 eq of fluorescein mercuric acetate (FMA) to beef heart cytochrome oxidase was found to inhibit the steady-state electron transfer activity by 50%, but further additions up to 10 eq had no additional effect on activity. The partial inhibition caused by FMA is thus similar to that observed with other mercury compounds (Mann, A. J., and Auer, H. E. (1980) J. Biol. Chem. 255, 454-458). The fluorescence of FMA was quenched by a factor of 10 upon binding to cytochrome oxidase, consistent with the involvement of a sulfhydryl group. However, addition of mercuric chloride to FMA-cytochrome oxidase resulted in an increase in fluorescence, suggesting that FMA was displaced from the high affinity binding site. Cytochrome c binding to FMA-cytochrome oxidase resulted in a 10% decrease in the fluorescence, possibly caused by Forster energy transfer from FMA to the cytochrome c heme. The binding site for FMA in cytochrome oxidase was investigated by carrying out sodium dodecyl sulfate gel electrophoresis under progressively milder dissociation conditions. When FMA-cytochrome oxidase was dissociated with 3% sodium dodecyl sulfate and 6 M urea, FMA was predominantly bound to subunit II following electrophoresis. However, when the dissociation was carried out at 4 degrees C in the absence of urea with progressively smaller amounts of lithium dodecyl sulfate, the labeling of subunit II decreased and that of subunit I increased. These experiments demonstrate that mercury compounds bind to a high affinity site on cytochrome oxidase, possibly located in subunit I, but then migrate to subunit II under the normal sodium dodecyl sulfate gel electrophoresis conditions. A definitive assignment of the high affinity binding site in the native enzyme cannot be made, however, because it is possible that mercury compounds can migrate from one sulfhydryl to another under even the mildest electrophoresis conditions.