It is well known that inflammatory aggression makes important changes in plasma proteins levels. Among all of these proteins, C3 fragment of the complement system and haptoglobin, Acute Phase Reactants Proteins, are considered important mediators of the inflammatory response although their specific behaviour is not completely understood. In this work we have studied the modification of both proteins after an inflammatory aggression, which was experimentally induced in rats, trying to connect their behaviour with the lesion stages. Our data show that the plasmatic level of both proteins is increased during the inflammatory aggression, although haptoglobin level gives a more accurate approach to the local phenomenons which occur in the inflammatory focus. This fact suggests that each acute phase reactant has a certain specificity depending on the kind of aggression suffered by a patient.