A study was undertaken to determine the relationship between the amino acid and crude protein content of maize using samples of yellow maize from the four main maize-producing areas of South Africa . After determining moisture, crude protein and amino acid content, the data were subjected to statistical analyses to determine to what extent protein quality changes with crude protein content, and whether this relationship differs between the four areas. All 17 amino acids studied, when expressed as 9 amino acid/kg dry maize, increased with crude protein content. But when amino acid concentrations were expressed as 9 amino acid/16 g N only four (aspartic acid, threonine, methionine and histidine) did not change with crude protein content, four others (glycine, lysine, arginine and cysteine) showed a negative relationship, whilst the remaining amino acids increased in concentration as the crude protein content increased. Of economic and nutritional significance to the balanced feed industry are the relationships between isoleucine, leucine, lysine and arginine with crude protein content. Equations to predict the content of these amino acids from the amount of crude protein in maize are given. The remaining amino acids can be estimated without loss of accuracy from their mean value expressed as g/16 g N. Although the relationship between certain amino acids and crude protein differed between certain areas, it is unlikely that these differences are more than empirical statistical observations, so there is little scope in attempting to improve the accuracy of these relationships by considering the areas in which the maize is grown.