BACKGROUND: This paper reports investigations of the extraction of transglutaminase (TGase) produced by Bacillus circulans BL32 on solid-state cultivation in order to obtain a crude extract with the highest possible specific activity. The optimization of downstream processing parameters for the effective recovery of the enzyme was carried out using response surface methodology based on the central composite rotatable design (CCRD) to reduce losses in the cultivated solids and to obtain a crude extract as concentrated as possible. Several solvents and temperatures were tested, followed by a 2(3) factorial design performed to optimize conditions extraction time, mechanical agitation, and solid:liquid ratio. RESULTS: The mathematical model showed that solid:liquid ratio has a significant negative effect on transglutaminase recovery. The optimal conditions for extraction were: water as solvent at 7 degrees C; 5 min extraction time; agitation speed 250 rpm; and 1:6 solid:liquid ratio. Under these conditions the model predicts a maximum response of 0.291 U mg(-1) of protein of transglutaminase activity recovery, very closely matching experimental activity of 0.285 U mg(-1) of protein. TGase recovery achieved under the optimized extraction conditions, according to the CCRD, was 2.5-fold higher than that obtained under non-optimized conditions previously employed. CONCLUSION: Results show that TGase can be produced in cheap solid state cultivations and the optimization of its downstream processing parameters can improve enzyme recovery in crude extracts and may have important impacts on enzyme costs. (C) 2008 Society of Chemical Industry.