The effect of a dietary leucine imbalance on chickens was investigated. The two criteria considered were growth and antibody production following immunization with sheep red blood cells. Three groups of chickens were fed diets containing increasing levels of leucine. Group 1 was fed a control diet containing 17,4 g leucine/kg, whereas Groups 2 and 3 were fed diets containing 25,6 and 37,8 g leucine/kg respectively. Five weeks after the dietary treatments had been introduced chickens were immunized, followed 4 weeks later by a further immunization. Antibody production following primary and secondary immunization was monitored at 3 – 4 day intervals. The average body mass of the chickens in each group was determined at weekly intervals. The dietary imbalance of leucine was shown to produce a marked depression in growth rate and a significant reduction in the level of serum IgM and IgG in birds in Group 1 and 2. The significance of these findings is discussed.