||In 1964 Mertz, Bates and Nelson ( 1) found that the mutant gene opaque-2 of maize produces a modified amino acid balance of the grain endosperm protein with increased quantities of lysine and tryptophan. This discovery opened up exciting new vistas for plant scientists and human and animal nutritionists. Feeding trials with diets of only opaque-2 or floury-2 maize, another high lysine mutant di'scovered subsequently, have confirmed that animals and humans, particularly the young, can gain weight very much faster than if fed normal maize diets. The implications of this discovery were immediately obvious to many scientists and the biochemical, physiological and genetic mechanism of the amino acid mutants in maize came under detailed study. The discovery of the high lysine maize mutants has provided the possibility of combining high quality protein into superior maize lines which can be used within the traditional agricultural structure of extensive areas of the world. The descriptive names "opaque" and "floury" indicate the dull, lusterless and chalk-like physical appearance of the kernels by which these mutants were originally isolated and identified. These characteristics represent an obstacle to acceptance by farmers accustomed to growing flint and dent types with their clean, shiny and lustrous appearance. In those areas where the floury grain types are traditionally grown such as the high Andean regions of South America, this would not be a problem, however, floufy types are not grown in the majority of the maize areas of the world either for human or animal food. The identification of high lysine lines within maize populations depends on careful chemical analysis. Until quite recently, most of the identification was made either by selecting the opaque kernels, which unfortunately precludes any opportunity to obtain better grain type, or by chemical analysis of small bulk samples which tended to obscure useful genetic variability containing the desirable combination.Several chemical methods have been recommended in the past but some of them are complicated and laborious, therefore not suitable for screening large populations, and others do not effectively indicate the levels of amino acid, i.e. in an opaque-2 segregating population in .which certain modifying genes are acting. In an attempt to provide simple techniques suitable for screening large numbers of small samples, different analytical methods already available for the determination of tryptophan and lysine, have been studied and reevaluated at the Protein Quality Laboratory of the International Maize and Wheat Improvement Center ( CIMMYT), and the Biochemistry Laboratory at Purdue University. Following this reevaluation, the following recommendations are made for routine evaluation of populations and segregating material produced in maize breeding programs.