Gliadin, the alcohol-soluble protein fraction of wheat, contains the factor toxic for coeliac patients. The numerous components of gliadin can be classified according to their primary structure into omega 5-, omega 1,2-, alpha- and gamma-type. Both omega-types have almost entirely repetitive amino acid sequences consisting of glutamine, proline and phenylalanine. alpha- and gamma-type gliadins contain four and five different domains, respectively, and are homologous within the domains III and V. Unique for each alpha- and gamma-type is domain I, which consists mostly of repetitive sequences rich in glutamine, proline and aromatic amino acids. Coeliac toxicity of gliadin is not destroyed by digestion with gastropancreatic enzymes. In vivo testing established the toxicity of alpha-type gliadins and in vitro testing of gliadin peptides revealed that domain I of alpha-type gliadins is involved in activating coeliac disease. The sequences -Pro-Ser-Gln-Gln- and -Gln-Gln-Gln-Pro- were demonstrated to be common for toxic gliadin peptides. Most of the in vivo and in vitro studies of synthetic peptides confirmed the importance of one or both of these sequences. Cultivated hexaploid, tetraploid and diploid wheat species do not differ significantly in potential toxic sequences of alpha-type gliadins.