During the submerged fermentation of Aspergillus oryzae medium concentration and the combination of soybean and wheat flours determined protease production. Without the addition of any chemicals low medium concentration led to conidiation and substantial increase in protease production because of low medium viscosity and subsequently higher dissolved oxygen concentration. The optimal flour combination depended on the starch content of the wheat flour. If the medium for maximum protease production was defined as balanced medium, unbalanced medium, either supplied with extra carbon or nitrogen source, stimulated conidiation to levels above 200 million conidia ml(-1) broth. The optimal supplement of NaCl and urea promoted protease production. The addition of CaCO3 and MgSO4 improved protease stability and repressed conidiation. Inoculum size and conidium age displayed few effects on protease production. Using conidia from submerged fermentation as inoculum similar protease production and conidiation to those using conidia from solid surface incubation were achieved. (C) 2003 Elsevier Ltd. All rights reserved.