Green fluorescent protein ( GFP) is an excellent biosensor as a result of its ability to be easily monitored in a wide variety of applications. Enzymes and proteins have been used as biological indicators to evaluate the immediate efficacy of industrial procedures, such as blanching, pasteurization, and disinfection treatments, as well as to monitor the satisfactory preservation of a product subjected to disinfection or sterilization. The purpose of this work was to study GFP stability in chlorinated water for injection ( WFI) and chlorinated buffered solutions at various pH ranges, with and without agitation, to evaluate the exposure time required for chlorine to decrease 90% of its fluorescence intensity ( decimal reduction time, D-value, min, 25 degrees C). Fluorescence intensity ( Ex/Em(max) = 394/509 nm) was measured immediately after the addition of GFP ( 8.0-9.0 mu g/mL) into buffered or unbuffered chlorine solutions with or without constant stirring. With solutions constantly stirred, GFP fluorescence decreased abruptly on contact with chlorine in concentrations greater than 150 ppm, with D-values between 1.3 min ( 147 ppm chlorine) and 1.7 min ( 183 ppm chlorine). In phosphate buffered chlorine solutions ( pH = 7.15 +/- 0.08), GFP retained its structure between 52 and 94 ppm, but protein stability decreased 10-fold when exposed to 110 ppm chlorine. The recovery of GFP fluorescence intensity due to renaturation was observed between 30 and 100 ppm chlorine in WFI ( final pH = 11.01 +/-0.23) without stirring. Stirring enhanced the contact between GFP and chlorine throughout the assay and provided a more accurate D-value evaluation. GFP performed as a suitable fluorescent marker for monitoring disinfection effectiveness.