Avian reovirus capsid protein (TB was genetically fused with a histidine (His(6)) tag and a UV-optimized green fluorescent protein (GFPuv) and expressed in Sf-9 cells. The fluorescence of GFPuv allowed for easy identification of protein localization and revealed that the fusion protein was quite stable in the cell culture. The fluorescence intensity (FI) exhibited a linear relationship (r(2) = 0.93) with the recombinant protein yield and therefore allowed for on-line tracking of the expression profile, which revealed an extremely high maximum yield of 70 mug per 106 cells. The recombinant protein was purified via immobilized metal affinity chromatography (IMAC) and a high purity (85%) was achieved in one step. During the purification, the fluorescence again enabled qualitative and quantitative monitoring of when and how much the desired product was eluted. The GFP-tagging strategy eliminated the need for cumbersome and time-consuming assays (e.g. Western blot or ELISA) for product analysis, thus GFP is an effective non-invasive on-line marker for the expression and purification of recombinant proteins in the baculovirus expression system. (C) 2004 Elsevier Inc. All rights reserved.