A new apparatus is described which can be used to investigate the in vitro antibacterial activity of antibiotics as a function of different concentration-time curves. The apparatus can be adjusted to simulate the biexponential serum level curves observed in vivo after oral or intramuscular administration. Preliminary studies were carried out with cefazolin against an E. coli strain simulating initial concentrations of 5, 10 and 20 micrograms/ml that decreased exponentially with half-lives of 30, 60 and 120 min. In all the situations tested there was an initial phase of rapid bactericidal activity followed by a phase of bacteriostatic activity, whose length depended on the drug elimination rate but was relactively independent of the initial concentrations. Bacterial regrowth occurred when the antibiotic concentration fell below the minimum inhibitory concentration (MIC) of the drug. The antibacterial activity of cefazolin, cephacetrile, and cephradine against an E. coli strains was also investigated, in a medium containing 4% human albumin, simulating the serum level curves observed in humans after an intramuscular dose of 1 g. The results obtained confirm that, for cephalosporins, the dosage schedule should be adjusted taking into account the potency of the drug (MIC) and its rate of elimination.