Phosphoprotein phosphatase obtained from chick embryo readily dephosphorylates casein which is characterized by sequences of O -phosphorylserine residues and by a high content of glutamic acid and aspartic acid. Dipeptides incorporating these amino acids and (SerP) 3 were synthesized. The phosphopeptides, equivalent to 380 μg phosphorous in acetate buffer (pH 5.8), were dephosphorylated by acid phosphatase but not by phosphoprotein phosphatase. Possible minimum structural features of a peptide required for phosphoprotein phosphatase activity are discussed.