ERYTHROMYCIN is the generic name of an antibiotic derived from Streptomyces erythreus. Its discovery, method of production, and isolation as a crystalline material were described in June, 1952, by McGuire and co-workers.1Erythromycin is a basic compound soluble to the extent of 2 mg. per milliliter in water. It is extremely soluble in organic solvents, such as alcohol, acetone, chloroform, and ethyl acetate.1The crystalline material is grayish white and is stable at room temperature. The pH of a saturated solution of erythromycin is approximately 9. Aqueous solutions in high and low concentrations are stable for eight weeks at 4 C. in a frozen state. At room temperature there is some deterioration during the first week, but none has been demonstrated in the ensuing seven weeks. The antibacterial activity of broth solutions begins to decline on the fourth day. After this there is continuous progressive deterioration. Heating