Pharmacodynamic in vitro models that simulate serum antimicrobial concentrations provide more information about the activity of an antibiotic than MICs or traditional time-kill methods. The aim of this study was to compare two pharmacodynamic simulation models using ATCC strains of five different species and five antibiotics. In the first model (Centriprep-10 system), a filtration-centrifugation process was used to eliminate the antibiotic; in the second model (microfiltration system) no centrifugation was necessary. The antibiotic concentrations tested were similar to those in serum after normal doses of cefuroxime, clarithromycin, ciprofloxacin, gentamicin and cefotaxime. No significant differences were observed in the killing rates between the models except in the case of Haemophilus influenzae and cefotaxime. The new microfiltration model had the following advantages: lack of the carry-over effect, the absence of centrifugation that could damage bacteria and the possibility of increasing the number of incubation periods to give a better fit of the kinetic profile of man.