A cyclomaltohexaose (alpha-cyclodextrin) determination method which is both highly reproducible and selective is described. It involves the formation of an inclusion complex between the cyclodextrin and methyl orange under conditions of low pH (1.2) and low temperature (16 degrees C) and is useful for the assay of cyclodextrin glucanotransferase activity. The formation of the inclusion complex causes a decrease in absorbance of the methyl orange solution and this is monitored at a wavelength of 505 nm. The decrease in absorbance is linearly correlated with the cyclomaltohexaose concentration in the range of 0.25 optical density unit and 0.30 mM cyclomaltohexaose. The specificity of the test for cyclomaltohexaose is high, with only limited interference by linear oligosaccharides and other cyclodextrins: cyclomaltoheptaose and cyclomaltooctaose cause absorbance variations of 16 and 5%, respectively, of the response of maltohexaose. The formation of the complex is instantaneous and the complex is stable in time, provided the temperature is constant. The presence of methyl orange does not hinder enzymatic activity determination. The reaction is stopped by acidification and absorbance is measured at the fixed temperature of 16 degrees C. Possible interferences inherent to the composition of the sample itself can be suppressed by running appropriate controls and calculating a corrected optical density. This colorimetric method is simple and should be versatile in assaying diverse cyclomaltohexaose glucanotransferase enzymes.