The extraction optimization of xylanases produced in solid-state cultivations of a strain of Bacillus circulars recently isolated from Amazon basin was examined, with the purpose of reducing enzyme losses in order to obtain crude extracts as concentrated as possible. Substrate for growth and enzyme production was an industrial fibrous soy residue, rich in protein and hetnicellulose. Several extraction solvents and temperatures were tested. A 23 factorial design was performed to find the best conditions of time, agitation and solid/liquid ratio. Maximum recovery was obtained by extracting in water at 7 degrees C, 40 min, 150 rpm and 1:6 solid/liquid ratio. The central composite design (CCD) used for the analysis of treatment combinations showed that a second-order polynomial regression model was in good agreement with experimental results, with R-2 = 0.90 (P < 0.05). Results show that optimization of the extraction conditions is a simple way of obtaining concentrated enzyme extracts from solid-state cultivation of industrially important microorganisms. (c) 2005 Elsevier Ltd. All rights reserved.