  We report on the study of thermal neutron flux using monitors based on
mixture of ZnS(Ag) and LiF enriched with a lithium-6 isotope at the deep
underground laboratory DULB-4900 at the Baksan Neutrino Observatory. An annual
modulation of thermal neutron flux in DULB-4900 is observed. Experimental
evidences were obtained of correlation between the long-term thermal neutron
flux variations and the absolute humidity of the air in laboratory. The
amplitude of the modulation exceed 5\% of total neutron flux.
