A conditional seedling lethal, monogenic recessive, endosperm mutant is described. Phenotypic can be accomplished when embryos are cultured in vitro on media supplemented with proline. The efficiency of the repair is proportional to the concentration of proline in the medium. Normal growth is resumed at a dose of 160 mg/l. All the data collected are most easily interpreted by assuming that the mutant, symbolized pro has a genetic block in the biosynthetic route leading to proline.This is probably the first case of a strict genetic requirement for an amino acid to be reported in Zea mays. The possible reasons for the difficulties encountered in isolating obligate auxotrophs in higher plants are briefly reviewed.