SummaryThe author describes a model to calculate the slope of the regression line of any antibiotic agent from its molecular weight.It deserves again notice that considerable variations exist among authors on the subject of critical concentrations, thus accounting for diverging results in sensitivity testing by different methods.The author is of the opinion that the clinician should have a better knowledge about the pharmacokinetics of the different antimicrobials, and he shows by means of a table the tissue concentrations of the most common antibiotic agents.