  We consider the so-called semiclassical variant of general relativity, where
gravitational field is not quantized but matter is quantized, for the simplest
composite quantum body - a hydrogen atom. We create a stationary electron
quantum state in the atom in the absence of gravitational field and study its
time evolution in the presence of the field, using the local Lorentz invariance
property of spacetime. It is shown that this state with a definite energy in
the absence of gravitational field is not anymore a stationary state in the
field. Therefore, quantum measurements of passive gravitational mass of
electron in a hydrogen atom can give the following quantized values, $m_n = m_e
+ E_n/c^2$, where $m_e$ is the bare electron mass and $E_n$ is its energy level
in the atom. We discuss some difficulties in the possible experimental
observations of this mass quantization phenomenon.
