An in vitro assay is described using saliva-treated bovine enamel slabs for determining the potential of chemotherapeutic agents to adsorb to tooth surfaces and act against plaque-forming bacteria. Chlorhexidine was found to inhibit the formation of in vitro plaque by Actinomyces viscosus, A naeslundii, Streptococcus mutans and S sanguis. Actinobolin was found to have marked antibacterial properties but limited adsorptive qualities.