Several changes occur in the properties of cAMP-dependent protein kinase during the differentiation of lens epithelial cells, and/or in the course of aging of fiber cells. Two types of isoenzymes (I and II) were found on DEAE-cellulose chromatography of epithelial cells. The chromatography of the crude extract of cortical fibers yielded one protein kinase fraction only, in the position of type I kinase. A sharp cAMP-dependent protein kinase peak was observed in the position of isoenzyme II on DEAE, and several lower molecular weight, cAMP-dependent protein kinase fractions appeared in Sephadex G-200 after mild acid treatment (pH 4.8) of the cortical crude extract. <sup>3</sup>H-cAMP saturation curve (Scatchard plot) of acid-treated protein kinase showed the loss of positive cooperativity. Aggregation of the I and II isoenzymes during lens cell differentiation is suggested.