A method for detecting proteins under mutual interaction which comprises
mixing a protein having a label for detection synthesized by a cell-free
protein synthesis method with another protein having a modification for
separation, or synthesizing a protein having a label for detection and
another protein having a modification for separation in a single system
by the cell-free protein synthesis method; separating a pair of proteins
formed by the interaction between these proteins with the use of the
modification for separation as described above; and distinguishing these
proteins with the use of the label for detection as described above. A
screening method using this method.