Lyophilised hemodialysates of uremic patients were fractionated on Sephadex G-15 for investigation of potential uremic toxins. One of the oligopeptides found in the fraction of postulated toxins was purified to homogeneity by gel, reversed-phase and adsorption chromatography and by high-performance liquid chromatography. Dansyl Edman degradation revealed the sequence Ala-Phe-Phe-Gly-Glu, which was synthesized by the solid-phase method. The comparison of synthetic and natural hexapeptide by chromatographic methods, mass spectrometry, 1H- and 13C-nuclear magnetic resonance proved the correct sequence. The peptide showed only weak activity in immunological test systems and in the lymphocyte proliferation test.