Another method is the fluorometric detection of gfp-labelled cells in solution. This method can be useful for screening, or confirmation of cell growth or survival in environmental samples since it is less sensitive than other methods. For example, Burlage et al. (1996) reported a detection limit of 10 CFU/ml Pseudomonas putida GFP-marked cells from soil samples suspended in phosphate-buffered saline and in our laboratory a detection limit of 10 CFU/ml Pseudomonas fluorescens GFP-marked cells from pure culture was determined (unpublished results). In our laboratory, we have investigated use of fluorometric detection for most-probable-number (MPN) enumeration of bacteria using both a standard spectrofluorophotometer for 1–3 ml samples in cuvettes and a fluorescent microplate reader for 96-well microtitre plates. Depending on the application, researchers have choices in the experimental approach they wish to take.