Four systems for a continuous cyclodextrin glucanotransferase (CGTase) production by Bacillus circulans ATCC 21783 were studied: free cells in a stirred reactor; free cells in a bubble column reactor; agar-entrapped and membrane-immobilized cells in a bubble column reactor. The performance of the process with free cells led to a higher enzyme level, specific CGTase activity and enzyme productivity compared to systems using immobilized cells. However, the residual enzyme activity (after approximately 300 h process) was from 96 to 97% for immobilized cells and from 78 to 86% for free cells. The four used systems provided a significantly higher CGTase yield (from 3.4-fold to 4.7fold) and productivity (from 1.4-fold to 2.9-fold) in comparison to known strains of Bacillus applied for the same aim. The concomitant high specific CGTase activity indicated that the established cultural conditions provided the dominant synthesis of the active protein. (c) 2005 Elsevier Ltd. All rights reserved.