Supragingival plaque samples were examined for alpha-amylase activity before and after cultivation. Amylase activity was determined by disappearance of 14C-labeled starch in a phosphate buffer, pH 6.9. In all plaque samples alpha-amylase activity was observed. On an average 75% of this activity was soluble, while the rest was bound to various plaque components. Known inhibitors of human alpha-amylase inhibited the enzyme activity in the plaque samples almost totally. The electrophoretic patterns of alpha-amylases in the plaque and human saliva samples were identical. Bacteria cultivated from the plaque samples showed no or low alpha-amylase activity. The results indicated that most of alpha-amylase activity in supragingival plaque samples are of salivary origin. The greater part of the enzyme activity is extracellular in the plaque, may be located at the plaque surface, and only a minor part is bound to the cells or to the insoluble components in the plaque.