The diffusional isotope effect of a monatomic Lennard-Jones liquid is calculated by molecular-dynamics simulation. Mass differences of 10-40 % were used. At equilibrium density, with decreasing temperature a strong reduction of the isotope effect is found that indicates a marked increase of the collectivity of motion. Changing the density at constant temperature, the same effect is seen that shows that the density change is the main driving force behind the reduction of the isotope effect.