A quantitative in vivo method for detecting protein-protein interactions will enhance our understanding of protein interaction networks and facilitate affinity maturation as well as designing new interaction pairs. We have developed a novel platform, dubbed "yeast surface two-hybrid (YS2H)," to enable a quantitative measurement of pairwise protein interactions via the secretory pathway by expressing one protein (bait) anchored to the cell wall and the other (prey) in soluble form. In YS2H, the prey is released either outside of the cells or remains on the cell surface by virtue of its binding to the bait. The strength of their interaction is measured by antibody binding to the epitope tag appended to the prey or direct readout of split green fluorescence protein (GFP) complementation. When two alpha-helices forming coiled coils were expressed as a pair of prey and bait, the amount of the prey in complex with the bait progressively decreased as the affinity changes from 100 pM to 10 microM. With GFP complementation assay, we were able to discriminate a 6-log difference in binding affinities in the range of 100 pM to 100 microM. The affinity estimated from the level of antibody binding to fusion tags was in good agreement with that measured in solution using a surface plasmon resonance technique. In contrast, the level of GFP complementation linearly increased with the on-rate of coiled coil interactions, likely because of the irreversible nature of GFP reconstitution. Furthermore, we demonstrate the use of YS2H in exploring the nature of antigen recognition by antibodies and activation allostery in integrins and in isolating heavy chain-only antibodies against botulinum neurotoxin.