Twenty children, while hospitalized with pneu monia, were treated by randomization with graded dosages of oral ampicillin; five each received 50, 100, 150, or 200 mg/kg daily in four equally divided doses for at least five days. The peak serum levels rose from a mean of 1 mcg/ml with 50 mg/kg per day to 4 to 7 mcg/ml with 200 mg/kg per day. Mild to moderate diarrhea, encountered in two of the five patients of each study group, was apparently unrelated to dose, and did not hinder absorption of the drug to a clinically significant degree.