1. Blood pyruvate carboxylase (pyruvate-CO2 ligase (ADP-forming); EC 126.96.36.199; PC) activities in young chickens and turkeys given low-biotin diets supplemented with biotin at graded levels were studied in three experiments.
2. In both species PC activity was related positively to the supplemental biotin level. The relationship was sigmoid and maximum activity was attained with supplemental levels above those required to give maximal growth response.
3. Enzyme activity decreased between 2 and 4 weeks of age but remained almost constant thereafter.
4. Activity in chicks was not affected by alterations in the fat or protein content of the diet.
5. Changing poults from high to low and from low to high supplemental biotin levels resulted in reversals in the levels of enzyme activity.
6. It is concluded that blood PC activity is a promising new criterion for assessing the biotin status of young chickens and turkeys.