An in vitro method for measuring barley protein digestibility is presented. Samples were first incubated with pepsin in HCl; pancreatin was then added concomitantly with a bacteriostatic borate buffer. After TCA-precipitation, soluble nitrogen was measured. The digestion was unaffected by accumulated free amino acids. There were no free amino acids following pepsin treatment, but the essential ones were well liberated by pancreatin. Results for barley grown in the field or in pots, and for decortified barley fractions agreed with true digestibility values determined with rats. Of these samples, the field-grown barleys per se differed too little for the accuracy to be confirmed. The other cereals tested, oats, rye, maize, wheat, and rice, gave unsatisfactory results with pepsin/pancreatin, and also with pepsin, pancreatin, or pronase used separately. The ranking of the cereals according to in vitro digestibility depended on the type of enzyme and on the enzyme-to-substrate ratio.