1. The maintenance requirement of adult male cockerels for isoleucine was measured by nitrogen balance. Measured amounts of a diet first-limiting in isoleucine were fed by tube each day for 6 d to give a range of intakes of from 0 to 100 mg isoleucine/kg body weight. A nitrogen-free diet containing energy, vitamins and both major and trace minerals was offered ad libitum during that time. Two series of diets were used, an unbalanced series, as described above, and a balanced series, in which synthetic L-isoleucine was added to each diet in the unbalanced series. Excreta were collected in bags during the last 3 d of the balance period and the N content of the excreta was analysed on wet, homogenised samples. 2. The resultant linear regressions were not statistically different for the unbalanced and the balanced series indicating that the response measured was to isoleucine. The pooled regression equation was: N retention = -144.14 (+/- 8.5) + 2.421 (+/- 0.19)I where I is the intake of isoleucine in mg/kg body weight day. The isoleucine required to maintain the body at zero N retention was therefore calculated to be 60 (+/- 3.2) mg/kg body weight day. 3. A case has been made for the expression of the amino acids required for maintenance to be in terms not of body weight but scaled according to the degree of maturity of the animal, and on this basis the amount of isoleucine required for maintenance was calculated to be 300 mg per unit of maintenance protein (Pm 0.73 mu) per day.