While the sugar content of various medicaments has been accepted as a hazard to dental health, the possible detrimental effect of the medicaments per se has been largely overlooked. In the present study the microhardness of dental enamel was examined before and after the exposure to various salts of penicillin, to tetracycline chloride, and to human saliva. The Vickers hardness index was found to1. remain stable or increase slightly in enamel exposed to saliva, non-aqueous penicillin suspensions and certain watery solutions of phenoxy-methylpenicillin calcium2. decrease to a moderate degree in most aqueous penicillin solutions, and3. decrease drastically in tetracycline solutions and in phenoxymethylpenicil-lin calcium solutions containing citrate.The results indicate that a direct reaction may take place between the dental enamel and the medicament and/or its degradation products when antibiotic compounds are takenper os.