A new phosphate analysis method is described which is designed primarily for use with routine phosphatase assays. The method is modified extensively from that of Fiske and SubbaRow [(1925) J. Biol. Chem. 66, 375] by decreasing 10-fold the level of reducing agent, ANSA, increasing the concentration of ammonium molybdate to 4 m m, utilizing HCl as the only acid present, and incorporating SDS. The method is insensitive to pH, ATP, proteins, and detergents at concentrations normally encountered in enzyme purification and analysis experiments. The color developed is fivefold more stable than that from the original Fiske-SubbaRow method, and the standard curve retains linearity for at least 24 hr with acid concentrations between 0.6 and 1.0 n HCl and for wavelengths between 500 and 900 nm. The simplicity, stability, and general applicability of the method allow for convenient and rapid analysis of large numbers of samples under a wide variety of conditions.