Mucoid Pseudomonas aeruginosa isolated from a patient with cystic fibrosis was cultivated at a slow growth rate (D = 0.05 h) under iron limitation in a chemostat. Biofilm was allowed to form on acrylic tiles. The kinetics of the biofilm formation was then investigated. The population of sessile bacteria reached 1.5 x 10(9) cells/cm2 on day 5 and remained relatively constant throughout the study (day 7). The population of planktonic cells in the chemostat reached 4 x 10(9) on day 1 and stayed fairly constant throughout. Planktonic cells were very sensitive to tobramycin. They were killed by exposure to 10 mg/l tobramycin within 2 h. Young biofilm cells of P. aeruginosa (day 2 of colonization) were found to be more resistant. Approximately 40% of the adherent cells remained viable after exposure to 10 micrograms tobramycin/ml for 5 h. An increase in the concentration of tobramycin to 20 mg/l resulted in an enhancement of the killing of young biofilm bacteria and approximately 1.5% of them remained viable after exposure to this concentration of antibiotic for 5 h. Old biofilm bacteria, examined at day 7, were the most resistant and 15% of the cells were found to be viable after they were exposed to 200 mg/l of tobramycin for 5 h. When either young or old biofilm cells of mucoid P. aeruginosa were scraped from the tiles to produce a planktonic cell suspension they were sensitive to 5 mg/l tobramycin.