The in vitro activities of purified potato starch branching enzyme (SBE) I and II expressed in Escherichia coli were compared using several assay methods. With the starch-iodine method, it was found that SBE I was more active than SBE II on an amylose substrate, whereas SBE II was more active than SBE I on an amylopectin substrate. Both enzymes were stimulated by the presence of phosphate. On a substrate consisting of linear dextrins (chain length 8-200 glucose residues), no significant net increase in molecular mass was seen on gel-permeation chromatography after incubation with the enzymes. This indicates intrachain branching of the substrate. After debranching of the products, the majority of dextrins with a degree of polymerization (dp) greater than 60 were absent for SBE I and those with a dp greater than 70 for SBE II. To study the shorter chains, the debranched samples were also analysed by high-performance anion-exchange chromatography. The products of SBE I showed distinct populations at dp 11-12 and dp 29-30, whereas SBE II products had one, broader, population with a peak at dp 13-14. An accumulation of dp 6-7 chains was seen with both isoforms.