This paper examines the properties in soluble form of two isoforms of starch synthase. One of these, granule-bound starch synthase I (GBSSI), is responsible for the synthesis of amylose inside the amylopectin matrix of the starch granule in vivo. The other, starch synthase II (SSII), is involved in amylopectin synthesis. Both isoforms can use amylopectin and malto-oligosaccharide as substrates in vitro. As well as acting as a substrate for GBSSI, amylopectin acts as an effector of this isoform, increasing the rate at which it elongates malto-oligosaccharides and promoting a processive rather than distributive mode of elongation of these compounds. The affinity of GBSSI for amylopectin as an effector is greater than its affinity for amylopectin as a substrate. The rate and mode of elongation of malto-oligosaccharides by SSII are not influenced by amylopectin. These results suggest that specific interaction with amylopectin in the matrix of the starch granule is a unique property of GBSSI and is critical in determining the nature of its products.