A pharmacokinetic model has been developed, by means of which all possible time courses of the concentrations of antibiotics in the plasma of treated individuals can be exactly simulated in vitro without diluting the test organism and affecting the growth curves. Equieffective concentrations in the system corresponded to the plasma concentrations in man produced by cefroxadine in a single oral dose of 250 mg and cephalexin and cephradine in a single oral dose of 500 mg.