Discoloration, pigmentation and even caries-like lesions have been reported in teeth subsequent to per oral administration of antibiotics. The present in vitro study examines the possible medicament/tooth reaction by exposing plane, polished enamel surfaces to aqueous solutions or oily suspensions of/various commonly used penicillin compounds, or to solutions of tetracycline sodium.The teeth were examined by visual inspection, microhardiness tests, scanning electron microscopy and microradiography. pH and |Ca2+of the test solutions were analyzed by potentiometry. Surface or subsurface lesions and a dec-lino in mierohardness were found in (most) enamel specimens exposed to aqueous solutions of the medicaments. Similar changes were found in test specimens immersed in deionized water, while saliva, oily suspensions of the medicaments and aqueous solutions made from ordinary Ca-peniciilin had no adverse effect on the enamel. The most serious destruction was seen in enamel specimens submerged in watery solutions made from effervescent (citrate-containing) (a-pcnicillin, or from tetracycline. A slight deeline in pH was observed in the aqueous solutions during the experiment. The calcium ion concentration did not increase parallel to enamel destruction.