A search for new leptons, charged and neutral, produced in e+e- collisions at an average c.m. energy of 44.2 GeV, has been carried out with the CELLO detector at PETRA. No such particles have been observed: we exclude pair production of new charged leptons at the 95% C.L. in the mass range 1.5-22.0 GeV/c2 and of neutral leptons in the mass range 3.1-18.0 GeV/c2. Single production of a heavy neutrino together with a standard neutrino has also been studied; we exclude an electron-like neutral lepton in the mass ranges 0.6-34.6 GeV/c2 for V-A coupling to the W boson, and 0.4-37.4 GeV/c2 for V+A coupling.