  We prove a separable reduction theorem for sigma-porosity of Suslin sets. In
particular, if A is a Suslin subset in a Banach space X, then each separable
subspace of X can be enlarged to a separable subspace V such that A is
sigma-porous in X if and only if the intersection of A and V is sigma-porous in
V. Such a result is proved for several types of sigma-porosity. The proof is
done using the method of elementary submodels, hence the results can be
combined with other separable reduction theorems. As an application we extend a
theorem of L.Zajicek on differentiability of Lipschitz functions on separable
Asplund spaces to the nonseparable setting.
