Many maize (Zea mays L.) mutant genes exist. Some affect protein content or composition, while others modify carbohydrates or kernel phenotype. In doublemutant lines, two mutant genes are present. We know little about interactions of such genes, however. We therefore examined a normal maize inbred, B37, 10 near-isogenic single mutants and 46 double mutants to analyze quantitative effects on alcohol-soluble endosperm proteins. Proteins were extracted with 70% ethanol0.5% sodium acetate-5% mercaptoethanol, and fractionated by reversed-phase high-performance liquid chromatography (RP-HPLC). Early peaks were alcohol-soluble glutelin (ASG) subunits, while late peaks contained zein. Results were quantified and statistically analyzed. In many double mutants, protein compositions differed significantly from averages of compositions of corresponding single mutants. For example, a high-methionine, water-insoluble ASG is absent when the opaque-2 (o2) gene combines with shrunken-1 (sh1) or surgary-1 (su1). Another water-insoluble ASG nearly doubled when floury-2 (fl2) andsu1 combined. A high-proline, high-histidine, water-soluble ASG nearly doubled in combinations offl2 witho2,su1 and sugary-2 (su2). Zein was about half its expected value wheno2 combined with amylose-extender (ae), floury-1 (fl1), soft-starch (h),sh1 andsu1. Thus, rapid protein extraction and quantitative RP-HPLC showed major new epistatic and synergistic effects of several mutant genes on protein composition. Unexpectedly, these effects often involve genes that primarily affect starch composition or kernel phenotype. Alcohol-soluble proteins often vary in amount, as ino2 lines. They also differ in nutritional value. Thus, RP-HPLC analysis of these proteins can identify nutritionally superior genotypes, and may help explain the basis of such quality.