In high-resoln. NMR, a network of scalar-coupled spins can be broken down into fragments which encode information about the chem. shifts and couplings that are relevant to a cross-peak multiplet in a two-dimensional correlation ("COSY") expt. Novel selective two-dimensional NMR correlation methods ("soft-COSY") are described which allow one to identify fragments on the basis of their topol. These new expts. allow one to "zoom in" on complicated cross-peak multiplets and include "passive spin filters" (PSFs), consisting of selective inversion pulses applied at the frequencies of passive coupling partners, i.e. spins that are not directly obsd. in the cross-peak multiplets under investigation. It is demonstrated how such PSFs can be inserted at various points in two-dimensional expts. and how such expts. can be characterized in terms of topol. The topol., couplings, and shifts of a fragment may be represented by a matrix M, while a selective expt. can be represented by a matrix P. Several degrees of Mand P can be distinguished. The techniques, which provide a novel method of discriminating and assigning spin systems, are demonstrated on a cyclopropane deriv. and the cyclic undecapeptide cyclosporin. [on SciFinder (R)]