AbstractThe pharyngeal flora was evaluated semiquantitatively by differentiating the grown colonies on blood agar plates, in 7 patients undergoing intravenous ampicillin treatment.Intravenous ampicillin in larger daily doses of 200 to 300 mg/kg caused marked changes that eradicated α‐streptococci in most cases, and gram‐negative bacilli appeared in 13 out of 14 cultures with high prevalences. Changes observed by smaller daily doses were less extensive, which indicated a does reponse reltionship.Staphylococcus aureuswas isolated with high prevalences in 3 of the total of 12 cultures in the course of ampicillin.Since these changes denote suppression of the normal flora and overgrowth af ampicillin‐resistant organisms in the respiratiory tract, which potentiate the development of supernifection, expected advantages must be weighed against these undesired effects, particularly when ampicillin is prescribed for long periods and larger doses. Culture of the pharyngeal swab would be a useful guide for the check of the bacterial over