Fluorescence behavior of pyrenyl groups bound to the anti-pyrenyl antibody was studied. The specific binding was found by an energy transfer from tryptophyl groups of the antibody to the pyrenyl group. The I-1/I-3 ratio of pyrenyl fluorescence indicated a nonpolar environment of the bound pyrenyl group, Fluorescence intensity and decay time, however, were not different from those in solution. Quenching of pyrenyl fluorescence by external quenchers was suppressed by the antibody when acrylamide or more bulky quenchers were used. These results indicate that the pyrenyl groups are not very tightly and not very deeply bound to the binding site and retain their inherent photophysical properties inside the binding site.