Inhibitors of trypsin (EC 184.108.40.206) and a-amylase (1,4-a-D-glucan glucanohydrolase, EC 220.127.116.11) were purified from seeds of rye and their complete and partial amino-acid sequences, respectively, were determined, in part by homology. The trypsin inhibitor was a single polypeptide chain of Mr 13753. Both proteins exhibited sequence homology with a group of cereal seed proteins that include inhibitors of proteinases and a-amylase. The trypsin inhibitor was most closely related to the barley trypsin inhibitor (76% identity) and the a-amylase inhibitor to CMa of barley (also an inhibitor of a-amylase activity) and to CMl1and CM2 of wheat (no known inhibitory activity). Antisera raised against the two inhibitors did not cross react, but the a-amylase inhibitor reacted with an antiserum raised against the 0.28 a-amylase inhibitor of wheat. The rye inhibitors had similar secondary structure contents with about 36-39% a-helix and 11-19% 13-sheet. These are the first amino-acid sequence and conformation studies reported for enzyme inhibitors from rye. Poly(A)-rich RNA from total polysomes, prepared from rye endosperms, was used as a template for cDNA synthesis and a cDNA library was constructed in AgtlO. The library was screened using two oligonucleotide probes which encoded two regions of the trypsin inhibitor (from amino-acids 38-42 and 44-48).One clone was isolated that hybridised to both probes. The nucleotide sequence of the clone AC(C.In) was determined. 1709 bp were sequenced showing an open reading frame that extended from the 5' end to 1621 and encoded a protein of 540 residues. The predicted amino-acid sequence showed striking sequence similarity to the serine/threonine SNFl subfamily of protein kinases with 62% and 48% identity, respectively, to the catalytic domains of SNFl and niml(^+). The functions of the SNFl subfamily are discussed.