Pharmacodynamics studies consider three main parameters: the size of the bacterial population, the concentration of the antibiotic and the duration of its action. The pharmacodynamic characteristics of colistin were studied in vitro with Escherichia coli. The bacterial kinetics were fitted using differential equations. The mathematical model gave qualitative and quantitative information about the characteristics of the antibiotic-bacteria association. Above all, when linked to a pharmacokinetic model, the model permitted the prediction of the drug's efficiency. Simulations of various dosage levels in which the administration route, dose size, or interval between doses varied, permitted a more rational optimization than a prediction of efficacy based on the time taken to achieve antibiotic plasmatic concentrations above the minimal inhibitory concentration. Pharmacokinetic/pharmacodynamic modeling seems to be an interesting possibility for determining antibiotic dosing levels during the preclinical phase.