In this report, we describe the catalytic properties of plasmin (Pln) adsorbed irreversibly onto a carbon paste electrode via its surface reaction with an electrogenic substrate (S-2644) synthesized specially for this purpose. Results show that the surface properties of Pln were retained in the potential range required for the electrochemical assay and under optimal conditions (pH 8.5 and 5 degrees C). Experimental data were linearized by using Lineweaver-Burk plots. The feasibility and the reliability of the assay was demonstrated from the linear relationships obtained with two different Pln-modified surfaces which led us to deduce the apparent Michaelis constant related to the catalysis. Homogeneous and heterogeneous values for K-m obtained using parallel photometrical and electrochemical assays under various conditions are given.