BACKGROUND: An inexpensive and readily available agroindustrial substrate such as rice bran can be used to produce cheap commercial enzymes by solid-state fermentation. This work investigates the production of food-grade proteases by solid-state fermentation using readily available Thai rice bran. RESULTS: A local strain of Aspergillus oryzae (Ozykat-1) was used to produce proteases. Rice bran used alone proved to have poor substrate morphology (insufficient porosity) for satisfactory solid-state fermentation. A certain amount of wheat bran was necessary to improve the morphology of the substrate. The following variables affected protease production: substrate composition, initial moisture content and initial pH. A high protease activity (similar to 1200 U g(-1) dry solids) was obtained on a substrate that had a wheat bran to rice bran ratio of 0.33 by dry weight, a moisture content of 50%, initial pH of 7.5, and incubation temperature of 30 degrees C. CONCLUSION: Nutritionally, rice bran used alone was as good a substrate as mixed bran for producing protease, but rice bran had poor morphological characteristics for consistent fermentation. A substrate that had a wheat bran to rice bran ratio of 0.33 by dry weight was best for producing protease. (C) 2008 Society of Chemical Industry.