A method has been developed to immobilize papain on cotton fabric by means of sol-gel technique. The activity of free papain and papain in silica sol under sonication was studied. Scanning electron microscopy, energy dispersive spectrometer and the Bradford method were used to characterize papain immobilization. The efficiency of the immobilization was investigated by examining the relative enzymatic activity of free and immobilized papain, respectively. The results show that the optimum pH value in the medium for immobilized papain is shifted to alkaline side. In addition, the adaptability of papain to environmental acidity is significantly increased. The thermostability of immobilized papain shows no significant change compared to the free enzyme. The papain immobilized on fabric by sol-gel technique retains more than 30% of the original activity after six reuses continuously. (c) 2006 Elsevier Inc. All rights reserved.