1 It is well-known that considerable variability and unpredictability in serum concentrations results from orally administered erythromycin. 2 Disposition kinetics and their variability were studies in 24 healthy subjects after a single dose of erythromycin lactobionate and four doses were studied to evaluate dose-related variability in five other subjects. 3 Erythromycin kinetics were adequately described by a classical two compartment open model with little intersubject variability. 4 Dose-related variability occurred. Clearance was independent of dose but T1/2 beta and Vdss increased with dose. 5 Data are presented to show that non-invasive sampling of urine and saliva are of limited value in studying erythromycin pharmacokinetics.