When testing the plaque-inhibiting effect of enzyme preparations in dentifrices the difficulty arises how to differentiate between enzyme inhibition and mechanical effect of brushing. To differentiate between these two effects, in the present investigation the influence was assessed of frequent scorings (3/week) on patient behaviour related to the plaque-reducing effect of 3 toothpastes (patient’s usual one, test, and placebo paste). The tests were performed double blind on 9 subjects over a 50-day period. The results of a factorial analysis of variance showed the plaque-reducing effect (p < 0.05) of the active toothpaste for all plaque sites in the Silness-Löe Index system, as well as its influence on the health of the surrounding gingiva. From the results it may be concluded that these reducing effects were caused by the enzyme activity of the amyloglucosidase and glucose oxidase in the test toothpaste.