The occurrence of a thermally induced first-order transition affecting the amidolytic activity of plasmin adsorbed onto bare and protein-modified graphite and glassy carbon was demonstrated in the 10-45 degreesC temperature range in the presence of a chromogenic substrate. Modification of the surfaces was achieved upon spontaneous adsorption of plasmin to surfaces bearing a coating of fibrinogen, whether electrochemically oxidized or not. The amount of fibrinogen adsorbed at graphite was determined by ELISA. The kinetics of the transition was characterized by its starting temperature (T-c), which was between 14 and 19 degreesC, the first-order rate constant, and the activation energy E-a deduced from Arrhenius plots. Results showed the absence of a correlation between T-c, E-a, and contact angle variations. It is therefore likely that these variables address separate steps in a complex pathway of reactions undergone by plasmin under mild thermal constraints, (C) zool Academic Press.