1. The activity of liver phosphorylase b from several mammalian species has been studied. The enzyme from rat or mouse has a higher activity than the rabbit enzyme, which is itself more active than pig liver phosphorylase b. 2 The activity of liver phosphorylase b is influenced by anions and by AMP, and these effects are influenced by pH. Fluoride, which is currently added to the assay mixture of phosphorylase a in crude preparations, is about as active as sulfate as a stimulator of phosphorylase b. 3. When assayed at pH 6.1 and in the presence of 0.15 M NaF, the activity of rat liver phosphorylase b reaches 25% of that of the a enzyme; if 1 mM AMP is also present, this value rises to 50%. 4. Methods are described that allow the determination of liver phosphorylase a without interference of b, and the determination of total phosphorylase (a+ b) in rat liver.