Domains of orientationally ordered streptavidin molecules bound to biotinylated amphiphile monolayers at the air/water interface can be compressed reversibly by a factor of 2 without destroying the order. By X-ray reflectivity it is shown that the water content of the protein domains at low lateral pressure corresponds to that of 2D crystals studied previously after transfer on solid support. The domains are highly compressible since 50 vol % of water can be removed upon compression. The results support the view that the protein forms a mesophase and not a crystalline phase.