Two peptides corresponding to bread wheat A-gliadin fragments 31-43 and 44-55, well known for their ability to damage the coeliac disease intestinal mucosa both in vitro and in vivo, have been confirmed to be very active in inducing in vitro agglutination of K 562 (S) cells. Removal of six amino acid residues from the carboxy-terminal end of the 31-43 peptide, or of five amino acid residues from the amino terminal end of the 44-55 peptide, resulted in a lower, but still very significant, cell agglutination activity. The peptide consisting of ten amino acid residues with a molecular mass of 1157.5 Da, isolated from durum wheat gliadin, was able to prevent agglutination of K 562 (S) cells induced not only by prolamine peptic-tryptic digests from all the cereals toxic in coeliac disease (i.e. bread wheat, rye, barley and oats), but also by the 31-43 and 44-55 peptides. The ability to protect K 562 (S) cells from agglutination was exhibited to the fullest extent also by all the peptides derived from the 1157.5-Da peptide by five progressive deletions of the terminal carboxylic residue, whereas the sixth consecutive deletion yielded a completely inactive peptide. A similar total loss of activity was observed upon addition of a glycine residue to the amino terminal residue of the 1157.5-Da peptide and all the above-mentioned active peptides derived from it. The remarkable sequence homologies existing between peptides able to induce [Gln-Gln-Gln-Pro and -Pro-Ser-Gln-Gln-] or to prevent [H2N-Gln-Gln-Pro-Gln-Asp-COOH] induction of cell agglutination strongly suggest that all these peptides compete for identical or structurally related binding sites on the cell surface.