Testing superstring-inspired E(6) models at linear e+e- colliders

It is shown that in a large class of E(6) models, either an e+e- collider operating at $\rs \geq 1.5$ TeV must find a signal for the production of exotic leptons, or a collider operating at $\rs \geq 300$ GeV must find at least one light neutral Higgs boson with a large invisible branching ratio. The region of parameter space where neither of these signals is visible can be excluded because here the lightest neutral exotic lepton, which is absolutely stable, would overclose the Universe.