Pharyngeal flora were evaluated semi-quantitatively by counting and differentiating the colonies growing on blood agar plates into four major bacterial groups. In children they consisted mainly of alpha-streptococci, followed by smaller portions of Neisseria, and of gram-positive organisms other than alpha-streptococci. Gram-negative bacilli did not occur under normal conditions. Oral ampicillin in a daily dose of 50 mg/kg caused suppression of the alpha-streptococci, and promoted emergence of gram-negative bacilli. Oral cephalexin in the same dose caused changes in the same direction, but less extensively than ampicillin . Parenteral aminoglycoside antibiotics in usual dosage did not change the pharyngeal flora. Differential counting of the growth is a simple and useful tool to evaluate the ecology of pharyngeal flora. It serves to monitor overgrowth of resistant organisms in the respiratory tract.