Sucrose synthase (UDPglucose: d-fructose 2-α-d-glucosyl transferase, EC 2.4.1.13) has been purified from the plant cytosolic fraction of soybean (Glycine max L. Merr cv Williams) nodules. The native enzyme had a molecular weight of 400,000. The subunit molecular weight was 90,000 and a tetrameric structure is proposed for soybean nodule sucrose synthase. Optimum activity in the sucrose cleavage and synthesis directions was at pH 6 and pH 9.5 respectively, and the enzyme displayed typical Michaelis-Menten kinetics. Soybean nodule sucrose synthase had a high affinity for UDP (K  _m, 5 micromolar) and a relatively low affinity for ADP (apparent K  _m, 0.13 millimolar) and CDP (apparent K  _m, 1.1 millimolar). The K  _m for sucrose was 31 millimolar. In the synthesis direction, UDPglucose (K  _m, 0.012 millimolar) was a more effective glucosyl donor than ADPglucose (K  _m, 1.6 millimolar) and the K  _m for fructose was 3.7 millimolar. Divalent cations stimulated activity in both the cleavage and synthesis directions and the enzyme was very sensitive to inhibition by heavy metals.