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Spectra of electrons from beta decays of 144Ce–144Pr nuclei have been measured and analyzed in order to find a contribution from a heavy neutrino. For the neutrino with the mass mνH in the interval of 150–350 keV, a new upper bound |UeH|2 ≤ (2−5) × 10−3 at 90% C.L. has been found for the mixing parameter.