We present an efficient exptl. method to evaluate whether the effective cross-relaxation rate between a pair of spins vanishes when applying an off-resonance spin-lock field. It is shown that the cross-relaxation rate can be made to vanish even when the two spins concerned resonate at different offsets and experience significantly different tilt angles of their resp. spin-lock fields. This is verified exptl. using a sample of 15N-labeled human ubiquitin, through selective excitation of chosen amide protons. The results are relevant for the quant. interpretation of off-resonance ROESY expts. (c) 1999 Academic Press. [on SciFinder (R)]