Abstract The kinetic properties of rabbit muscle aldolase have been studied in solutions containing various amounts of poly( N-vinyl pyrrolidone) or poly(vinyl alcohol) of different molecular weights. At low polymer concentrations both polymers enhance the maximum rate of cleavage of fructuose 1,6-bisphosphate, whereas at high concentrations inhibition occurs. In the presence of polymers of low molecular weight excess substrate inhibits the enzyme reaction, a phenomenon not observed in the absence of polymers. The thermal denaturation of the enzyme is markedly increased by the presence of polymers. It is suggested that the principal effect of these polymers is most probable the alteration of the enzyme conformation.