We have constructed three plasmid vectors for the expression of green fluorescent protein (GFP) fusion proteins using the following motif: (His) 6 -GFP-EK-X, where X represents chloramphenicol acetyl-transferase (CAT), human interleukin-2 (hIL-2), and organophosphorous hydrolase (OPH), respectively, (His) 6 represents a histidine affinity ligand for purification, and EK represents an enterokinase cleavage site for recovering the protein-of-interest from the fusion. The CAT and OPH fusion products (โผ63 kDa GFP/CAT and โผ70 kDa GFP/ OPH) were expressed at 4.85 ยตg/mL (19.9 ยตg/mg-total protein) and 1.42 ยตg/mL (4.2 ยตg/mg-total protein) in the cell lysis supernatant, and, in both cases, enzymatic activity was retained while coupled to GFP. In the case of hIL-2 fusion (โผ52 kDa), however, the GFP fluorescence was significantly reduced and most of the fusion was retained in the cell pellet. Linear relationships between GFP fluorescence and CAT or OPH concentration, and with enzymatic activity of CAT or OPH, indicated, for the first time, that in vivo noninvasive quantification of proteins-of-interest, was made possible by simple measurement of GFP fluorescence intensity. The utility of GFP as a reporter was not realized without disadvantages however, in particular, an incremental metabolic cost of GFP was found. This could be offset by many benefits foreseen in expression and purification efficiencies.