A procedure is described for approximately 1000-fold purification of a nonspecific acid phosphatase from lupine seedlings. The preparation has both general phosphomonoesterase and pyrophosphatase activity. It shows either very little or no activity toward 5′-nucleotides, glucose 1-phosphate, trimetaphosphate, diphenyl phosphate, diphosphopyridine nucleotide, and ribonucleic acid. With p-nitrophenyl phosphate as substrate, the phosphatase shows optimal activity at pH 5.2–5.5. The Michaelis constant for the reaction is 3 × 10 −4 M. Fluoride inhibition of the hydrolysis is noncompetitive. Inactivation upon dilution or purification is prevented by the surface-active agent, Triton X-100. Ethylenediamine tetraacetate increases activity further. Of the cations tested, copper is the most inhibitory; manganese and magnesium are slightly stimulatory. Ethylenediamine tetraacetate prevents the inhibition by copper and augments the stimulation by magnesium. p-Chloromercuribenzoate has negligible effect upon enzymic activity at a saturating concentration of p-nitrophenyl phosphate; it is stimulatory, however, at low substrate concentrations. The differences of plant and animal phosphatases in substrate specificity and the kinetics of inhibition by fluoride may reflect differences in the in vivo functions and mode of action of these enzymes.