A cyclomaltodextrin glucanotransferase from alkaliphilic bacteria was purified to PAGE homogenous by ammonium sulfate precipitation and DEAE-cellulose DE-52 chromatography with 11.5 fold purification and 5.7% recovery. Mr estimated with concentration gradient PAGE was 151,000. The optimum conditions for activity were pH 7.0, temperature 60 degrees C, stable in the range 6.0-9.0 and below 50 degrees C. The enzyme activity was activated by salicin and maltitol, slightly inhibited by Sn2+, Mn2+, inositol and pullulan, strongly inhibited by Ag+, Cu2+, Al3+, Fe2+, Pb2+, Hg2+ and Zn2+. The maximum absorption was at 270 nm in ultraviolet spectrum, the maximum emission wavelenghth in the excitation spectrum was 283 nm, the maximum emission spectrum determined at 283 nm was 335 nm. The effects of some protein modification reagents on the CGTase activity have been studied. Histidine and tryptophan residues may be essential for activity, carboxyl groups might have some effects on the activity.