The complex of iron(III) coproporphyrinI (FeCPI) with antibody D5E3 was studied as an artificial peroxidase, using o-dianisidine as a substrate. At saturation with respect to antibody, the initial rates of a-dianisidine oxidation are practically the same for free and bound FeCPI at a concentration 5 x 10(-9)M, but the catalytic rate constant (k(c)) for bound FeCPI exceed (k(c)) for free FeCPI by two- to threefold. This difference can be explained by a real enhancement of (k(c)) at the antibody-active site. The dependence of initial rates of the reaction on substrate concentrations obeyed Michaelis-Menten kinetics and revealed substrate activation at high concentrations of o-dianisidine. A comparison of the Stern-Volmer constants for o-dianisidine-induced quenching of the porphyrin fluorescence proves that antibody-bound coproporphyrin is equivalently accessible to the substrate as protoporphyrin bound to apoperoxidase from horseradish peroxidase (HRP). Based on analysis of the (k(c)) dependence on H2O2 concentrations in the FeCPI-antibody system, we suggest that interaction with hydrogen peroxide is the rate-limiting step for the oxidation reaction.