1. Experiments were carried out to establish the conditions required for the measurement of the availability to chicks of biotin in feed ingredients by a bioassay based on the response of blood pyruvate carboxylase (EC 22.214.171.124; PC) activity.
2. Reference diets were formulated which gave a linear response in blood PC activity at 3 weeks of age over a wide range of supplemental dietary biotin concentrations.
3. Dietary protein concentration was found to affect blood PC activity. Hence purified ingredients in the reference diets were wholly or partially replaced by the test ingredients so that all diets in each assay contained the same amounts of protein, fat and metabolizable energy. Comparison of the blood PC activities of birds$ given the test diets with those given biotin-supplemented reference diets provided a measure of the available biotin content of the test ingredient.
4. Bioavailabilities of biotin were found to vary widely in the cereals and vegetable-protein sources tested.