The characteristics of starches from 17 endosperm mutant genotypes in a common Oh43 inbred background were examined by gel-permeation chromatography (GPC), iodine affinity (IA), and scanning electron microscopy (SEM). The chain-length distributions of amylopectins were determined by an enzymatic- chromatographic method. Each genotype exhibited distinctive GPC elution patterns of its native and isoamylase-debranched starches and distinctivemorphology as noted by SEM. The amylose-extender (ae), dull-1 (du1), and sugary-1 (su1) genes were associated with increased amounts of amylose and intermediate fractions compared with normal starch. The waxy (wx) gene was epistatic to other genes relative to the accumulation of amylopectin, which was consistent with work done elsewhere. The discrepancy in amylose percentage determined by GPC and IA in some genotypes may have resulted from the presence of a large amount of intermediate materials in those genotypes, which could not always be distinguished from amylose by the IA method. For example, in ae starch, most of the intermediate materials were measured as amylose by the IA procedure, whereas in du1, ae brittle-1 (bt1), and ae du1 starches, most of the intermediate materials were exclded from IA measurements. The intermediate fractions from each genotype in the GPC elution profiles also differed from each other, suggesting differences in molecular weight and/or branching. The proportions of long B chains and the average chain length of amylopectins were increased when the ae gene was present. In contrast, the du1 gene decreased the proportions of the long B chains of amylopectins. The mutants containing he ae gene showed low degrees of branching in amylopectin; mutants containing the du1 and/or su1 genes hd high degrees of branching. Genetic background played a major role in determining the fine structure of starch components. The effects of interactions between recessive mutant genes on the structures and morphology of different starch genotypes were evident.