The method described is capable of detecting less than 0.01 microgram/ml of erythromycin and/or erythromycin ethylsuccinate in serum. Recoveries of erythromycin and erythromycin ethylsuccinate, when added to the level of 0.6 microgram/ml, were 102 and 97% with relative standard deviations of 6.0 and 4.5%, respectively. Erythromycin ethylsuccinate in serum at 37 degrees was shown to degrade rapidly to erythromycin in a first-order rate. When stored at -20 degrees, however, only 10% of the erythromycin ethylsuccinate in serum was hydrolyzed even after storage for 36 days. The method was used to analyze erythromycin and erythromycin ethylsuccinate in sera from ten subjects administered with an oral dose of erythromycin ethylsuccinate. In addition to a small amount of anhydroerythromycin ethylsuccinate and 8,9-anhydro-6,9-hemiketal erythromycin ethylsuccinate, at least two other metabolites were detected in sera and one was tentatively identified as erythralosamine.