3-Hydroxymethylene-2-thioxopyrrolidine (HMTP), the major product derived from radish mustard oil, was studied for its activity to inhibit the growth of mutans streptococci, their in vitro plaque formation and their glucan production. The minimum inhibitory concentration (MIC) (800-1600 mg/l) of HMTP at pH 7.0 was reduced to 200 mg/l by lowering the medium pH to 5.0. A dose-dependent inhibition of in vitro plaque formation was observed at 200-800 mg/l dose of HMTP. Production of water-insoluble glucan (WIG) was effectively inhibited by 45-98%, depending on HMTP dose (200-800 mg/l), while only 22% inhibition of water-soluble glucan (WSG) production was observed at an 800 mg/l dose. Copyright © 2001 Elsevier Science B.V.