Genetic improvement of protein quality of maize endosperm to make it better balanced and of greater biological value both for human and animal consumption has received major emphasis in CIMMYT's maize breeding program for the last three years. This is being achieved through the use of the Opaque-2 gene which enhances considerably the levels of two limiting essential amino acids, namely, lysine and tryptophan. The mechanism involves the depression of the biochemical synthesis of the alcohol soluble zein fraction known to be extremely deficient in these amino acids and a concomitant increase in non-zein protein synthesis of such fractions as albumins, globulins and glutelins that are rich in lysine and tryptophan. Opaque-2 versions of normal maize materials of great diversity have been produced at CIMMYT and elsewhere but the commercial production of these high quality protein materials has moved at a slower pace. This is due to a number of problems which vary considerably in degree and magnitude in different parts of the world. These problems include lower grain yield, consumer non-acceptability of soft lustreless floury texture of Opaque-2 kernels, and greater susceptibility to ear rots. CIMMYT has concentrated its research efforts in combating these problems. Progress has been rapid, and many of these problems are being resolved. Also, useful basic information has been accumulated which will accelerate the rate of progress of breeding high quality protein materials with better agronomic types.