A case of nonreciprocal partial cross-incompatibility was observed in a genetic line used in the course of an experiment involving controlling elements. When the derivative genotypes from this stock were crossed as female parents with specific male parents, ears with reduced seed-set (RSS) were obtained;This RSS effect is a heritable character. It is hypothesized that the RSS incompatibility condition in the female parents is influenced by a cytoplasmic-chromosomal interaction. The incompatible condition of the specific pollen type is under the control of a chromosomal factor. The interaction between the incompatible female and pollen from incompatible source leads to the RSS effect. The site of this incompatibility interaction is not on the silk surface since the incompatible pollen does germinate and grow normally on the silk surface. It is concluded that the incompatibility interaction occurs either inside the silk or inside the ovule.