An integral equation technique is used to obtain an approximate formula for the apparent susceptibility of a small ferrite sphere of radius a within a microwave cavity. The first-order effect of a plane wall is also considered using the quasi-stationary approximation that the fields within the sphere are uniform. It is shown that for spheres at a distance of the order of a wavelength λ away from the cavity wall, the wall contributes to the apparent susceptibility of the sphere terms of the order of (ka)3 and higher, where k = 2π/λ, justifying to order (ka)3 the use of free space calculations in cavity problems to estimate the fields within the sphere.