Bovine lactoperoxidase (LPO) is taken as a model protein of mammalian peroxidases to investigate the activity and the stability of the enzyme in the presence of different surfactants. The cationic benzalkonium chloride (Bz) has proved efficient in preserving the enzymatic activity for over 10 days, while the native enzyme completely lost its activity within 3-4 days. The presence of Bz allows the enzyme to preserve its secondary structure for a long time, as shown in GD spectra, and creates a more hydrophobic environment for the enzyme, as indicated in fluorescence studies. Moreover, this surfactant at a concentration of 0.01% (0.3 mM) increases the lactoperoxidase activity in the first 2 h of incubation at 37 degrees C. Both hydrophobic and electrostatic interactions of the cationic surfactant seem to be responsible for the enzyme activation and stabilization, and this is a promising result in view of industrial applications of enzymes.