A new apparatus is described which serves 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 a cephalosporin derivative, cefazolin, against Escherichia coli and Klebsiella sp. strains simulating initial concentrations of 5, 10, and 20 mug/ml that decreased exponentially with half-lives of 30, 60, and 120 min. Surviving cells were counted at 1-h intervals for 10 h. 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 relatively independent of the initial concentrations. Bacterial regrowth occurred when the antibiotic concentration fell below the minimum inhibitory concentration of the drug against the strains tested. The antibacterial activity of cefazolin, cephacetrile, and cephradine against E. coli and Klebsiella 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 suggest that, for cephalosporins, a longer half-life might be more useful than higher peak levels.