We generalize a differential effective-medium approximation previously
developed for the effective linear response of a fractal cluster, to
treat the effective nonlinear response. We consider a material having
a nonlinear J-E characteristic which forms a cluster in another, possi
bly nonlinear material. A differential equation for the cluster-size-d
ependent nonlinear susceptibility is derived and a general solution is
obtained. Numerical examples are given for two special cases: a clust
er of a nonlinear material in a linear host, and a cluster of a linear
material in a nonlinear host. An enhancement in the nonlinear respons
e due to clustering is found for the case of clustering of a nonlinear
material in a linear host.