  The recent analysis of the normalization of reactor antineutrino data, the
calibration data of solar neutrino experiments using gallium targets, and the
results from the neutrino oscillation experiment MiniBooNE suggest the
existence of a fourth light neutrino mass state with a mass of O(eV), which
contributes to the electron neutrino with a sizable mixing angle. Since we know
from measurements of the width of the Z0 resonance that there are only three
active neutrinos, a fourth neutrino should be sterile (i.e., interact only via
gravity). The corresponding fourth neutrino mass state should be visible as an
additional kink in beta-decay spectra. In this work the phase II data of the
Mainz Neutrino Mass Experiment have been analyzed searching for a possible
contribution of a fourth light neutrino mass state. No signature of such a
fourth mass state has been found and limits on the mass and the mixing of this
fourth mass states are derived.
