The multiplicity of antibiotics currently available for the treatment of bacterial infections in outpatients and inpatients has led to considerable confusion and uncertainty in recent years. An attempt is made to establish indications for the use of the various groups of antibiotics, paying due regard to cost as well as other aspects. This means drawing a distinction between the respective needs of out- and inpatients. In treating outpatients, where resistance is less of a problem than in hospitals, use is made principally of oral substances. In the light of current knowledge the vast majority of bacterial infections occurring in ambulatory practice can be optimally treated with pyrimidine-sulfonamide combinations, penicillins, tetracyclines and erythromycin. In contrast, the range of broad-spectrum antibiotics administered in hospitals, for the most part parenterally, in infections of a life-threatening nature or due to organisms exhibiting multiple resistance, is quite different. Most important are the more recent penicillins and cephalosporins and the aminoglycosides. By combining betalactam antibiotics with aminoglycosides and attempt is made to extend the therapeutic spectrum and intensify antibacterial activity. A conclusive judgement on the therapeutic differences between the more recent penicillins and cephalosporins as single-substance therapy or in combination with aminoglycosides is not yet possible at present. There seems good reason, however, to combine the more recent penicillins with aminoglycosides in infections in which enterococci, pseudomonas and anaerobic bacteria are also involved. In infections in which there is a possibility of Klebsiella involvement or in which penicillin allergy or penicillin resistance is present, treatment tends to be a combination of cephalosporins and aminoglycosides.