A comparison of the product-inhibition patterns during cleavage of D-fructose 1,6-diphosphate by aldolases from yeast, rabbit muscle and Bacillus stearothermophilus shows an ordered reaction sequence for all three enzymes, with dihydroxyacetone phosphate the last-leaving product. Addition of Zn2+, Co2+, Fe2+, Mn2+ or Cd2+ ions to the inactive apo-(Bacillus stearothermophilus aldolase) restores activity to different extents, whereas Ni2+, Mg2+ or Cu2+ ions have no effect. The cleavage activity of this aldolase is not enhanced by added K+ ion. The effects of metal replacement on thermal stability, Km and Vmax. are given and the possible role of the metal is discussed in the light of these results.