Filtration stress, consisting in the rapid filtration ofAcer pseudoplatanusL. cell suspension cultures, resulted in significant differences between the peroxidases (EC 1.11.1.7) released during cell growth and those released after filtered cells were resuspended in fresh medium (recovery medium). These differences concerned mainly modifications of (i) the pH optimum of peroxidase activity (guaiacol as electron donor), (ii) the number and the pI values of the peroxidase isoenzymes as shown by isoelectric focusing, and (iii) the molecular weights of the different peroxidase fractions determined by gel filtration chromatography. The presence of 1 mMLi+in the recovery medium inhibited the release of peroxidase and this effect was partially reversed by K+. The release of peroxidase by stressed cells was also strongly inhibited by Na2CO3in the recovery medium. The results presented are consistent with the proposal that the characteristic isoperoxidase patterns induced by filtration stress might be used as a model to study the response of plant cells to stress.