M-GTFI, originally screened as an inhibitor of Streptococcus mutans glucosyltransferase, strongly inhibited alpha-glucosidase, in a non-competitive manner especially when the synthetic substrate p-nitrophenyl-alpha-D-glucopyranoside was used. It also inhibited beta-glucosidase, beta-amylase and, to a lesser extent, beta-glucuronidase. The inhibitor was stable in neutral and alkaline pH ranges and dependency of the inhibition on pH and temperature was not observed. Some proteinases and polysaccharides-hydrolyzing enzymes as well as human saliva did not inactivate the inhibitor. There was a correlation between the release of sulfate anions from the inhibitor molecule on incubation with HCl (0.2 N) at 100 degrees C and loss of inhibitory properties of the molecule. It is suggested that the presence of sulfate ester linkages in the inhibitor molecule play an important role in the inhibition process.