Erythromycin is known to be an effective form of oral therapy against many Grampositive organisms, including the pneumococcus. In recent years erythromycin has been altered in such a manner as to increase its rate and degree of absorption from the gastrointestinal tract and presumably to improve its clinical effectiveness. Stephens and Conine1 have shown that esters of erythromycin whose fatty acid portion contains 5 or more carbon atoms are poorly absorbed but that the fatty acids containing 2 to 4 atoms give higher blood levels than the parent antibiotic. The highest blood levels are obtained with the proprionate ester. Griffith et al.2 confirmed the presence of higher blood levels with propionyl erythromycin than with erythromycin base and also showed that the proprionate ester provides earlier and more prolonged "therapeutic" concentrations than does the base. Alteration of the molecule by the addition of the propionyl radical changes neither the