The "in vitro" rate of killing on different microorganisms was studied for three cephalosporins (cephazolin, cefuroxime and cefoxitin) and four aminoglycosides (gentamicin, tobramycin, sisomicin and amikacin). In all experiments an inoculum effect and a negative influence by adding plasma to the nutrient broth were observed, the latter phenomenon being more pronounced with cephalosporins than with aminoglycosides. While the "in vitro" effects of antibiotic concentrations equal to the MIC or to the MBC were informing on the intrinsic antibacterial activity of single drugs, a more satisfactory approach to therapeutic reality was provided by studying the "in vitro" effects of concentrations equal to peak blood level or to the logarythmic mean of blood level, that is a parameter which contributes to the blood-tissue diffusion gradient. Our data show that antibiotics provided with an higher peak blood level and a more sustained logarythmic mean of blood concentrations, and an higher ratio between these two kinetics parameters and MIC or MBC, are favoured in the experimental system here adopted.