A genetic system to display proteins as their active and functional forms on the cell surface of yeast, Saccharomyces cerevisiae, has been exploited. Surface-engineered (arming) cells displaying amylase or cellulase could assimilate starch or cellulose as the sole carbon source, although S. cerevisiae can not intrinsically assimilate them. Arming cells with a green fluorescent protein (GFP) from Aequorea victoria can emit green fluorescence from the cell surface in response to the environmental conditions. From these results, we attempted to construct a system to monitor the foreign protein production in yeast by simultaneous displaying the enhanced GFP (EGFP). The expression in yeast of the Escherichia coli beta-galactosidase-encoding gene was examined as an example of intracellular production and that of the human interferon-alpha (omega, IFN-omega)-encoding gene as an example of extracellular production. Their productions and the simultaneous surface-display of EGFP as a reporter were controlled by the same promoter, GAL1. The relationship among fluorescence signals and their productions was evaluated. The surface-display system, unlike one using tag-proteins, would be able to facilitate the monitoring of native protein productions in bioprocesses using living cells in real time by the combination of promoters and GFP variants.