A simple theory of the force between large spheres in a fluid is outlined. The effect of the solvent is included by using the Percus-Yevick theory for hard sphere mixtures to describe core exclusion forces, and including a layer of small dielectric constant near the spheres to simulate the alignment of the solvent dipoles in the vicinity of the spheres. The usual expression for the van der Waals force is used. The resultant force is very similar to the experimental results of J. N. Israelachvili and colleagues (Chem. Scr. 25, 7 (1985)). © 1986.