Potato starch amylose was hydrolyzed to varying degrees by incubation with barley beta-amylase for different periods. Determination of reducing sugars and gel-permeation chromatography showed that amylose fractions with different number average chain lengths (DP(n)BAR 40-610) were obtained. Enzyme-resistant starch (RS) was formed from the fractions in aqueous solutions (0.83%, w/v) at 4-degrees-C. Under the experimental conditions, the yield of RS increased with DP(n)BAR to plateau values of 23-28% within a region of DP(n)BAR of 100-610. X-ray diffraction showed a B-pattern for all RS samples obtained. The DP(n)BAR of the RS varied only between 19 and 26 and, thus, it is independent of the chain length of the amylose from which it was formed (DP(n)BAR 40-610). The results suggest that RS may be formed by aggregation of amylose helices in a crystalline B-type structure over a particular region of the chain (about 24 glucose units).