The first studies on the operational stability of cross-linked enzyme crystals (CLECs) in organic media are described. Although these catalysts display high initial specific activity, they inactivate rapidly, losing more than 50% of the initial activity within the first 4 h under continuous flow. Furthermore, the inactivation is not reversible when returned to an aqueous medium. The same rapid inactivation occurs with adsorbed protease preparations that show similar high initial specific activity (propanol-rinsed enzyme preparations (PREPs) of subtilisin and alpha-chymotrypsin).