The effects of La3+ on total activity and initial activity of RuBPCase (ribulose 1,5-hisphosphate carboxylase) in tobacco seedlings were studied. When treated with 20 similar to 80 mu mol.L-1 LaCl3 in Hoagland solution by water culture, the activities of RuBPCase were raised, and the activation action of LaCl3 on initial activity was higher than that on total activity. But when the concentration of LaCl3 surpassed 80 mu mol.L-1, the activities of RuBPCase were inhibited. It was also found that La3+ and Ce3+ can replace Mg2+ to activate the activity of RuBPCase in the enzymatic reaction, and the activation actions of La3+ and Ce3+ on the purified enzyme are higher than those on crude enzyme. The results indicate that La3+ activates the activity of RuBPCase in vivo as well as in vitro.