ABSTRACT.Cyclic AMP binding activity was determined in the ciliateTetrahymena pyriformisNT‐1 strain. The fractions having the binding activity were eluted in a single peak coincident with a protein kinase activity. Although metal ions were not essential for activity, the binding was slightly activated by Mg2+or Ca2+. The binding activity was sensitive to temperature, ionic strength, and pH of the reaction mixture and was decreased by treatment of the cytosol protein with trypsin or by heating at 100°C. The binding was specific for cyclic AMP, with an estimated apparent Kd of 40 nM. When the cyclic AMP binding activity in subcellular fractions was measured, an increase in the activity of ciliary, mitochondrial, and microsomal fractions was observed during the transition from the exponential to the stationary phase of cell growth, whereas no significant change occurred in the binding activity of the whole cell homogenate. These results suggest that the redistribution of cyclic AMP binding proteins may be implicated in the regulation of cyclic AMP concentration in the ce