The activity in vitro of tibezonium (Rec 15–0691), a new 1,5-benzo-diazepine derivative, has been investigated. The drug was found active especially against Streptococcus, Diplococcus and Corynebacterium strains which are agents of oropharyngeal diseases. The activity of tibezonium was pH dependent against Staphylococcus aureus SG 511 and Streptococcus pyogenes 821 (at pH 8.0–8.5 it was more active) and the presence of horse serum provoked a small decrease of the antimicrobial properties. No interference on the activity of the tibezonium has been found in presence of smokers and non-smokers sal