A pharmacokinetic model has been developed for erythromycin involving seven transfer or interaction steps. The major steps describing the role of erythromycin derivatives and dosage form design in the absorptive process, impact of protein binding on drug distribution, specificity of erythromycin for bacterial ribosomes, and the determination of serum half-life for various erythromycin salts and dosage forms, are discussed. Bioavailability data for several erythromycin products, including erythromycin stearate film-coated tablets (250 mg and 500 mg), erythromycin stearate oral suspension, erythromycin base enteric-coated and film-coated tablets, erythromycin ethyl succinate suspension and sachets, and erythromycin lactobionate i. v. are presented. Effects of clinical protocol and formulation differences on drug serum levels are briefly described. The authors express concern with the over-extrapolation of bioavailability results to clinical dosage regimens and use.