The extensive charge heterogeneity of maize (Zea mays L.) zeins observed in isoelectric focusing (IEF) (about 15 bands with pI's in the pH range 6-9) has been found to be independent of extraction procedures or of endosperm development. Zeins do not stain for glycoproteins and exhibit only one lipoprotein component, with pI 3, representing 3-5% of the total protein.Zeins are very resistant to in vitro deamidation, at both acidic and alkaline pH, at high temperatures, and for rather prolonged times. On the basis of the zein content in acidic and basic amino acids, and of the respective pI's exhibited in IEF (mostly in the pH range 7-8) it has been calculated that at least 90% of the glutamic and aspartic acids (≈52 residues out of a total of ≈ 190) are present as asparagine and glutamine.Amino acid analysis of zein fractions isolated by preparative IEF has demonstrated changes in the composition of 18 amino acid residues. However, since these changes affect only neutral and hydrophobic residues, it is concluded that the observed zein heterogeneity is partly based on in vivo deamidation of glutamine and asparagine and partly to spot mutations in some of the genes responsible for zein synthesis.