A two-compartment in-vitro model is described in which the compartments are separated by a hollow-fibre dialyser. It was used successfully to simulate the 2-compartment kinetics of intravenously administered ciprofloxacin and to observe the activity of ciprofloxacin against a strain of Serratia marcescens growing in the peripheral compartment. Because of the complexity of the apparatus, however, the experiments were labour-intensive, prone to break-down, and gave results only slowly. There was a possibility that drug-resistant variants were carried over from one experiment to the next by re-use of the dialyser. While the model could give very flexible kinetics and retained all of the culture, there were constraints on the variations in kinetics from the physical sizes of the compartments and practicable flow rates. An ever-present problem was the possibility of fluid diafiltering rapidly into either compartment. Such a model should perhaps only be used when a very short drug half-life is to be simulated or very flexible kinetics required. Simpler, multi-channel equipment could be more productive.