Aleurone tissue of mature wheat (Triticum aestivum L. cv. Sappo) grains make novel polypeptides in response to abscisic acid (ABA), but only in the presence of Ca(2+). Effects of ABA plus Ca(2+) include up- and down-modulation of other polypeptides. The ABA-induced polypeptides appear not to be the 21-kilodalton (kDa) amylase inhibitor which has been reported to be ABA-inducible in barley.Aleurone tissue from developing grains of different ages failed to respond to ABA plus Ca(2+) in any way. Endogenous ABA levels were determined by monoclonal radioimmunoassay in developing, mature, and "sensitised" developing tissues. The ABA level rose to a maximum at 35 days post anthesis but was not detectable in mature cells. Developing layers sensitised to gibberellic acid (GA) showed decreased levels of ABA, similar to those in mature tissue, concurrent with acquired responsiveness to GA in respect of its induction of α-amylase. However, these sensitised cells still remained non-responsive to added ABA in terms of modulation of polypeptide pattern, though they did respond to ABA in the blocking of GA-induced α-amylase production. The role of protein phosphorylation in signal transduction was examined. The implications of these findings are discussed with reference to the role of ABA in developing and mature aleurone cells.