  Experimental data on the number of neutrons born in the heavy water targets
of the large neutrino detectors are used to set the limit on the proton
life-time independently on decay mode through the reaction d -> n+?. The best
up-to-date limit tau_p > 4 10^23 yr with 95% C.L. is derived from the
measurements with D_2O target (mass 267 kg) installed near the Bugey reactor.
This value can be improved by six orders of magnitude with future data
accumulated with the SNO detector containing 1000 t of D_2O.
