  We study the role of the $\Delta$-isobar current on the response functions
for high energy inclusive quasielastic electron scattering from nuclei. We
consider a general Lagrangian which is compatible with contact invariance and
perform a fully relativistic calculation in first-order perturbation theory for
one-particle emission. The dependence of the responses upon off-shell
parametrizations is analyzed and found to be mild. A discussion of scaling
behaviour and a comparison with various non-relativistic approaches are also
presented.
