For the isolation of coeliac active peptide fractions the peptic tryptic digest of whole gliadin was successively separated by ultrafiltration, gel filtration, cation-exchange chromatography, anion exchange chromatography and high-performance liquid chromatography. After each separation step the peptide fractions obtained were characterized by amino acid analysis and examined for coeliac activity in an immunological test (LIF test) and in an organ-culture test. The most active fractions have molecular weights ranging from 7,000 to 14,000 daltons, high contents of Glx (greater than 40 mol-%), Pro (greater than 20 mol-%) and Phe (greater than 5 mol-%) and low contents of S-containing and basic amino acids (0 and less than 2.0 mol-%, respectively). The peptide fraction B3142 obtained after five separation steps seems to be a pure peptide, which shows activity in the immunological test for all coeliac patients examined in very low concentrations. This peptide consists of 53 amino acid residues and has the composition Glx24, Pro15, Val4, Phe3, Ser2, Leu2, Asx1, Gly1, Tyr1.