in which tagged antibodies can be functionally expressed. Furthermore, as the GFP11 tag is located at the opposite end of the binding interface, antigen-binding activity is preserved even when interacting with the complementary GFP1-10 fragment, as shown in figure 2. A first series of experiments was carried out to investigate the possibility of fusing the GFP11 peptide to the C terminus of scFvs. We constructed a vector based on our pEP series [24]. This contained the SV5, GFP11 and His6 tags fused to the C terminus of the scFv, with restriction sites (BssHII and NheI) compatible with our phage display vector, pDan5 [27], allowing rapid transfer following antibody selection by phage display (figure 1). The levels of scFv-GFP11 recovered after expression is comparable to the normal recovery of the same scFv-D1.3 without the GFP11 tag. The complementation between scFv-D1.3-GFP11 and GFP1-10 completely restores GFP fluorescence, conferring a highly stable fluorescence to the scFv, without interfering with the antigen binding activity of the single chain, as shown by the ELISA (figure 2) and FLISA experiments (figure 5). Comparing FLISA (figure 5) and ELISA (figure 2) carried out similarly, shows that ELISA is approximately twice as sensitive, able to detect down to ,2.5 nM, while FLISA has a limit of detection of ,5 nM). Once complementation has been carried out, detection using FLISA and GFP complementation does not require additional secondary reagents. Furthermore, as fluorescence is quantitative, it does not require protein purification, and providing the GFP1-10 is in molar excess, can be used to directly determine the amount of a GFP11 tagged protein in a crude mixture, as shown in figures 4. We were also able to show that GFP complementation can be used References 1. 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