Effectiveness of Chloramine T trihydrate (N-chloro-p-toluene sulfonamide) on both planktonic and sessile populations of different Legionella pneumophila strains was assessed. Although Chloramine T is a recommended commercial formulation for disinfecting cooling towers, there is a lack of published data about the efficacy of this compound against both planktonic and sessile populations of L. pneumophila. Planktonic L. pneumophila strains were suspended in tap water and sessile L. pneumophila strains were grown on stainless steel which is used in the construction of cooling towers, followed by exposure to the biocide. The sensitivity of both planktonic and sessile populations of L. pneumophila strains was different. The biocide was found effective below recommended dosages (0.1-0.3%) against planktonic populations of L. pneumophila, whereas it was determined that higher dosages than those recommended were required for sessile populations of L. pneumophila. The results indicated that studying only the planktonic populations of L. pneumophila for biocide tests might not be sufficient to provide information about the optimum dosage and contact time. Therefore, efficacy has to be tested on both planktonic and sessile bacteria. (c) 2006 Elsevier GmbH. All rights reserved.