Modifier genes have been described that convert the soft endosperm of opaque2 mutants to a hard, vitreous phenotype. The mode of action and the components of the genetic system involved in this seed modification are poorly understood. We used genetic and biochemical analyses to investigate the number of opaque2 modifier genes, their mode of action and their relationship to the biochemical alterations in the modified endosperm. Using two inbred opaque2 lines, we showed that the activity of opaque2 modifier genes is influenced by the genetic background. Analysis of segregating progenies and recombinant inbred lines derived from crosses between opaque2 and modified opaque2 genotypes indicated two independent loci affecting seed opacity and density. Consistent association between endosperm modification and enhanced accumulation of the gamma-zein storage protein suggested that either this protein is directly involved in the process of seed modification, or else that a modifier gene could be tightly linked to the genes responsible for gamma-zein synthesis.