Amylopectin (AP) and intermediate materials (IM) from five endosperm mutant genotypes in a common Oh43 inbred line were isolated and examined by gel-permeation chromatography, iodine affinity, blue value (BV), and viscosity. The chain-length distributions of AP and IM were determined using an enzymatic- chromatographic method. The degrees of branching in AP and IM decreased when the amylose-extender (ae) gene was present. The dull-1 (du1) gene produced AP and IM with the highest degrees of branching among the samples. The ae starch had a significantly (P less than 0.01) longer peak average chain length (CL) of the long-B chains in the IM fraction (177 glucose units) than did the AP faction (73 glucose units) or the other starches (37-56 glucose units). The higher iodine affinity in ae starch of the IM (6.1) compared with that of the AP (2.8) supported the idea that the IM had a longer CL than did the AP. There were no significant differences in the peak CL of A or B chains in AP and IM fractions of brittle-1 (bt1), du1, ae bt1, and ae du1 starches. The IM of ae and ae du1 starches had higher BV than did the AP fractions; however, the IM of du1 and ae bt1 had lower BV than did the AP fractions. The limiting viscosity number and gel- permeation chromatography results indicated that the AP and IM fractions of bt1 and du1 starches possessed more branching and larger hydrodynamic volume properties than those of the ae, ae bt1, and ae du1 starches. The present study demonstrated that genetic background affects the CL of starch branches, degree of branching, and iodine binding properties of starches.