In this study we purified both acid phosphatases (Apase) secreted from tomato and lupin roots, and compared the properties of these two enzymes. The secretory Apases from tomato and lupin showed the following similar properties. 1) Both enzymes were homodimers consisting of two identical subunits, each with a molecular weight of approximately 68 kilodaltons (kD) in tomato and 72kD in lupin. 2) The enzymes were stable in the pH range of 4-9. 3) The enzymes showed an optimum temperature of 37-40℃ for their activity and were stable at temperatures below 60℃. 4) The enzymes exhibited a omparable affinity for p-nitrophenyl phosphate (the apparent Km values were 2.7-3.0×10-5M). On the other hand, there were some slight differences in the isoelectric point, optimum pH, specific activity, substrate specificity, and inhibitory effect of metal ions between the two enzymes. herefore, it was considered that the most important difference in the root ability of the two plants to hydrolyze organic phosphorus may depend mainly on the amount of Apase secreted from a unit amount of roots.