  We have considered the properties of anisotropic two-leg ladder models with
S=1/2 or S=1 spins on the rungs, using White's density matrix renormalization
group method. We have generalized the method by taking into account the
symmetries of the model in order to reduce the dimensions of the matrix to be
diagonalized, thereby making possible to consider more states. The boundaries
in the parameter space of the extended region, where the Haldane phase exists,
are estimated.
