Total serum levels estimated in children ofter orag administration of either 500 mg azidocillin or 250 mg ampicillin. The means of the total levels of azidocillin after 0.5 and 1 h were higher than similar data obtained in adults. No such difference was found with ampicillin. Correction of the means of the total serum levels over the first 2 h for differences in the extent of protein binding of azidocillin and ampicillin gave free serum levels which were comparable for both antibiotics. The coefficients of the free serum levels and the MIC’s of both antibiotics for bacterial species causing bronchial infection indicate that the ratio and the amounts of the antibiotics advocated are adequate for the prevention of such infections.