A search for a non-minimal Higgs boson produced in the Higgsstrahlung
process and decaying into undetectable particles is performed using the
data collected by the L3 experiment at center-of-mass energies up to 209
GeV at LEP.  The decays of the Z bosons into $q\overline{q}$, 
$e^+e^-$ and $\mu^+\mu^-$ are considered. Lower limits on the Higgs mass 
are set as a function of the Higgs branching ratio into invisible particles 
assuming the cross-section for the Higgsstrahlung process.