ADRIANI, O (reprint author), INFN, SEZ FIRENZE, I-50125 FLORENCE, ITALY.
We search for neutral heavy leptons that are isosinglets under the standard SU(2)L gauge group. Such neutral heavy leptons are expected in many extensions of the standard model. Three types of heavy leptons Ne, N(mu), N(tau) associated with the three neutrino types nu(e), nu(mu), nu(tau) have been directly searched for and no evidence for a signal has been found. We set the limit Br(Z0 --> nu(l)N(l)) < 3 x 10(-5) at the 95% CL for the mass range from 3 GeV up to m(Z).