Acciarri, M (reprint author), IST NAZL FIS NUCL, VIA CELORIA 16, I-20133 MILAN, ITALY.
A search for unstable sequential neutral and charged heavy leptons has been made at center-of-mass energies 130 and 136 GeV with the L3 detector at LEP. The neutral leptons are assumed to decay via mixing to electrons and muons. No evidence for their existence was found. We exclude unstable Dirac neutrinos for masses below 59.3 (57.9) GeV and unstable Majorana neutrinos below 48.6 (47.2) GeV if the neutrino couples to the electron(muon) family. We exclude unstable charged heavy leptons for masses below 61 GeV for a wide range of the associated neutral lepton mass.