A survey of 46 varieties of cereals and related species (including 27 different species from the Poaceae) indicated the presence of a strong inhibitor of wheat a-amylase in all seven Hordeum species tested. Rye contained a lower level of inhibitor activity, but the other species contained insignificant amounts of wheat a-amylase inhibitor activity. The partially purified barley inhibitor was most effective in inhibiting wheat cr-amylase activity at high pH. The addition of chromosome 2 of barley to wheat (Chinese Spring addition line 2H) resulted in an apparent increase in the molecular weight of the a-amylase produced during germination. This was probably due to the formation of a complex between the inhibitor encoded by the asi gene on chromosome 2 of barley and wheat a-amylase 2. Breeding of wheat with the barley inhibitor gene may reduce the impact of the high a-amylase levels that result from pre-harvest sprouting in wheat.