An experiment to check the standard dispersion law for ultracold neutr
ons is described. The experiment is based on searching for a shift of
the resonance line of a neutron interference filter as the neutron vel
ocity component parallel to the filter surface is varied, The first re
sults attest to a statistically significant effect. No mimicking effec
ts were found in a control experiment, but their possible existence ca
nnot be completely ruled out at present. (C) 1998 American Institute o
f Physics. [S0021-3640(98)00610-0].