  In this paper we prove the monotonicity of positive solutions to $ -\Delta_p
u = f(u) $ in half-spaces under zero Dirichlet boundary conditions, for
$(2N+2)/(N+2) < p < 2$ and for a general class of regular changing-sign
nonlinearities $f$. The techniques used in the proof of the main result are
based on a fine use of comparison and maximum principles and on an adaptation
of the celebrated moving plane method to quasilinear elliptic equations in
unbounded domains.
