The gain or loss of stability of three flavoprotein oxidases, glucose oxidase, lactate oxidase, and glycolate oxidase, upon immobilization in a hydrated silica gel by a sol-gel process was quantified. Glucose oxidase (isoelectric point, IP, pH 3.8) retained most or all of its initial activity, while lactate oxidase and glycolate lost most of theirs. The half-life of glucose oxidase at 63 degrees C increased upon immobilization 200-fold; the half-lives of lactate oxidase and of glycolate oxidase were not extended beyond those of the water-dissolved enzymes. After lactate oxidase (IP pH 4.6) was electrostatically complexed with the weak base poly(N-vinylimidazole) prior to its immobilization, most of its activity was retained and its half-life at 63 degrees C increased 150-fold. Lactate oxidase was also stabilized when electrostatically complexed with the stronger base poly(ethyleneimine) prior to immobilization. Glycolate oxidase (IP pH 9.6) was not stabilized by poly(N-vinylimidazole) but was stabilized by poly(ethyleneimine) complexing prior to immobilization. The complexed enzyme retained its initial activity upon immobilization, and its half-life at 60 degrees C also increased 100-fold. The results show that encaging an oxidase in a silica gel can lead either to gain in stability or to loss of activity and that electrostatic complexing is required for stabilization by encapsulation of some, but not all, flavoprotein oxidases.