This review discusses the correlations between in vitro antibiotic sensitivity test results and in vivo clinical situations. It is concluded that the relations between blood levels and minimal inhibitory concentrations of antibacterial drugs cannot always be applied as criteria of sensitivity. Important exceptions are local treatments and treatment of infections in body sites where antibiotics are concentrated. Examples of agreement and lack of agreement between in vitro test results and treatment results in infections caused by bacteria with acquired resistance are given. Finally, the special problem of the interpretation of results obtained in tests with intestinal bacteria and antibacterials used for growth promotion is discussed.