Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)The main use of green fluorescent protein (GFP) is as a reporter system, where the existence of the protein is usually determined visually using fluorescent microscopy. Although fluorescence-based quantification of GFP is possible, background fluorescence in plants and in plant extracts was observed by our group. Another phenomenon we observed that makes quantification difficult is the increased level of GFP fluorescence in Nicotiana benthamiana leaf extracts, probably the result of dimerization of GFP molecules promoted by interaction with some component(s) of tobacco extracts. In the current work, the background fluorescence was minimized and the enhancement of GFP fluorescence in tobacco extracts was eliminated with the addition of urea to the measured solution so that a simple quantification assay for the GFP in the tobacco extracts could be established. (c) 2009 Elsevier Inc. All rights reserved.3921811Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)