Partial purification of acid pyrophosphatase activity from dried red kidney beans was achieved. The crude enzyme was found to adhere to plastic and was very unstable. These problems were solved by extraction with low pH and high-ionic-strength buffers. This extraction procedure separated acid pyrophosphatase activity into three parts. One of these activities appears to correspond to the purple phosphatase isolated by other workers (1—3). The other two fractions showed both general phosphomonoesterase and pyrophosphatase activity, but were most active with pyrophosphate and were used for further characterization. The pH optimum for the enzyme was approx 5.5-6.0 with pyrophosphatase, and it exhibited substrate inhibition with pyrophosphate and ATP at low pH. The partially purified acid pyrophosphatase was estimated to be a dimer of approx 98 kDa (mol wt estimated by gel filtration on Sephacryl S-200) with no detectable carbohydrate or iron content. Of the cations tested for their effect on pyrophosphatase activity, iron was the most inhibitory, followed by magnesium and zinc.