Abstract Much has been written concerning the high arginine requirement of chicks fed diets based on casein as the source of protein. The peculiar amino acid balance of casein, as suggested by studies from this laboratory (Fisher, Shapiro and Griminger, 1960), may in part explain this phenomenon. We now wish to draw attention to a recently published article on seasonal variations in the arginine content of casein, which might provide another missing link in explaining the high arginine requirement of chicks on casein diets. According to Kiermeier and Kirchmeier (1963) the amino acid content of milk casein changes considerably with the season. The authors found, for example, a large increase in essential amino acids between July and November, and a corresponding decrease in nonessential amino acids. Specifically, the value for arginine ranged from 3.5 to 4.8% of the protein during the year. This could mean a difference in excess of 30%…