Tools for selective recognition and sensing of kinase-catalyzed phosphorylation at tyrosine or serine/threonine residues located on the protein surface are essential for understanding signal transduction cascades of living cell. A stable complex of RNA and peptide (ribonucleopeptide) provides a convenient approach to tailor a receptor for small molecules. In vitro selection of an RNA-derived pool of ribonucleopeptide afforded a ribonucleopeptide receptor specific for phosphotyrosine. The phosphotyrosine-binding ribonucleopeptide receptor discriminated phosphotyrosine against tyrosine, phosphoserine and phosphothreonine. © 2005 The Surface Science Society of Japan.