Dehydroascorbate reductase has been isolated from spinach chloroplasts and purified to apparent homogeneity. The N-terminal amino acid sequence of the enzyme is homologous to the Kunitz-type trypsin inhibitors from plant sources. It is shown that spinach DHA reductase and soybean trypsin inhibitor are both capable of reducing dehydroascorbate when in the reduced (thiol) form but acquire trypsin-inhibiting activity in the oxidized (disulfide) state. Reduced chloroplast thioredoxins also reduce dehydroascorbate.