The hypothesis that surrogate planktonic pathogens (Bacillus cereus and polystyrene microspheres) could be integrated in biofilms and protected from decontamination was tested. Pseudomonas fluorescens biofilms were grown on polyvinyl chloride coupons in annular reactors under low nutrient conditions. After biofilm growth, B. cereus spores and polystyrene microspheres (an abiotic control) were introduced separately. Shear stress at the biofilm surface was varied between 0.15 and 1.5 N m(-2). The amount of surrogate pathogens introduced ranged from approximately 10(5) CFU ml(-1) to 10(10) spheres ml(-1). The quantity of surrogate pathogens integrated in the biofilm was proportional to the amount introduced. In 14 of the 16 cases, 0.4-3.0% of the spores or spheres introduced were measured in the biofilms. The other two cases had 10% and 21% of the spores detected. Data suggested that the spores germinated in the system. The amount of surrogate pathogens detected in the biofilms was higher in the mid-shear range. Chlorine treatment reduced the quantity of both surrogate pathogens and biofilm organisms. In one experiment, the biofilms and B. cereus recovered when the chlorine treatment was terminated. Planktonic surrogate pathogens can be integrated in biofilms and protected from chlorination decontamination. This knowledge assists in understanding the impact of biofilms on harbouring potential pathogens in drinking-water systems and protecting the pathogens from decontamination.