  We examine the gravitational properties of Lamb shift energies. Using
available experimental data we show that these energies have a standard
gravitational behavior at the level of $\sim 10^{-5}$. We are motivated by the
point of view that Lamb shift energies may be interpreted as a consequence of
vacuum fluctuations of the electromagnetic field. If this is the case, our
result is a test of the gravitational properties of quantum fluctuations. The
result is of interest in relation to the problem of the zero-point energy
contribution to the cosmological constant. Indeed, the problem presupposes that
the zero-point energy gravitates as all other forms of energy, and this
supposition is what we test.
