A new assay for glycogen phosphorylase (EC 188.8.131.52) is presented. This assay employs a filter paper technique in which newly formed glycogen-14C is precipitated on paper squares using 66% ethanol while other radioactivity is removed by washing. It is rapid and reproducible and may be employed with crude as well as purified enzyme preparations. An additional advantage of the method is the potential for increased sensitivity. It should find utility for assay of extremely small tissue samples and should be useful in kinetic studies.