The recommended dosage of erythromycin, without regard to the formulation prescribed, for children with streptococcal pharyngitis is 30 to 40 mg/kg/day. We previously reported an acceptable streptococcal eradication rate among patients with pharyngitis treated with erythromycin estolate, 20 mg/kg/day. In this study, an extension of the earlier one, the efficacy of this same dosage of erythromycin estolate was compared with a 40 mg/kg/day dosage of erythromycin ethylsuccinate. Streptococcal eradication rates were nearly identical in the two groups of patients. The efficacy of erythromycin estolate at a dosage lower than that recommended for children is most likely explained on pharmacologic grounds: better absorption and higher levels in serum and tissue than those achieved with other erythromycin formulations. It seems rational to calculate required dosages of erythromycin on the basis of the formulation being administered.