A simple in vitro model for culture of biofilm populations of self-bioluminescent Pseudomonas aeruginosa was used for real-time monitoring of the effects of ciprofloxacin. Biofilms of these organisms were established within Sorbarod filters, perfused with a chemically defined simple salts medium. The biofilm population was shown to achieve a pseudo-steady state which was reproducible and stable over several days. The viability of membrane-associated and eluted cells was assessed by spread plate viable counts and by monitoring bioluminescence as a measure of metabolic activity. Pseudo-steady state biofilms were exposed to 5x MIC ciprofloxacin (0.3 mg x l(-1)) in the perfusing medium for 1 h. Whilst both methods for viability assessment indicated an immediate reduction in viable cell numbers, the decline recorded with bioluminescence was greater. The use of bioluminescent bacteria proved to be a rapid and sensitive method for the measurement of real-time antibacterial effects on a bacterial biofilm.