Cephalosporins are being given more and more frequently empirically as initial therapy, until the bacteriological findings become available. The wide selection of cephalosporins available make the choice of the most suitable one difficult for the clinician. Cephazolin, cephradine and cephacetrile were tested against three standard bacteria strains (Escherichia coli, Staphylococcus aureus, Kelebsiella pneumoniae) in normal broth and with the addition of 30% inactivated human serum; the geometric mean of the minimum inhibitory concentration (MIC) was determined by serial dilution, and the minimum bactericidal concentration (MBC) by inoculation onto solid medium. On the addition of 30% serum, cephazolin showed a reduction in effectivity of up to 360% for the MIC and up to 74% for the MBC, cephacetril up to 250% for the MIC and 100% for the MBC, and cephradin a maximum loss of 16%. Successful treatment of a severe infection requires a high drug level or a corresponding tissue titer. Due to the high tissue concentration of cephradin and the unrivalled stable antibacterial effectivity in serum, this antibiotic can be recommended for clinical use.