Unwanted micro-organisms in mice can jeopardize experimental protocols and should be eliminated if a colony becomes infected. The opportunistic bacterium Pseudomonas aeruginosa has been shown to cause infection in mice. In order to eliminate this bacterium from a specific-pathogen-free full barrier mouse area at our animal facility we treated mice with 7 ppm chlorine for 7 weeks and then with 10 ppm chlorine in the drinking water. The P. aeruginosa status of mice was examined by agar culture and PCR. The results show that 6 months after commencement of the treatment the colony was free of P. aeruginosa.