Acid phosphatase (EC 126.96.36.199) from yellow lupin (Lupinus luteus) seeds was purified to homogeneity by ammonium sulphate fractionation, affinity chromatography, cation-exchange chromatography, gel filtration or reverse-phase HPLC. The enzyme is a dimer with the 50 kD and 44 kD subunits and contains 7.3% of carbohydrate, forming at least four oligosaccharide chains. The optimum pH for the enzyme is 5.4. The apparent Km for p-nitrophenyl phosphate was estimated to be 0.28 mM and Vmax = 1780 IU/mg of protein. The purified phosphatase has the highest specific activities reported for any plant acid phosphatases measured for any native or synthetic substrate. The enzyme has broad specificity; however, cyclic nucleotides, pyrophosphate or phytate are not cleaved. It is inhibited by molybdate, fluoride and phosphate. There is no change in the enzyme activity in the presence of EDTA, phenanthroline and tartrate.