One-dimensional NOE expts. applicable to labeled macromols. are presented which allow the manipulation of specific spin diffusion pathways and thus unambiguously identify clandestine spins through which the direct NOE is mediated. A treatment of spin diffusion using av. Liouvillian theory is shown to describe adequately these phenomena. Expts. are carried out on an 15N-labeled sample of human ubiquitin. (c) 1999 Academic Press. [on SciFinder (R)]