Selective single-transition cross-polarization (ST-CP) may be achieved in four-level systems such as encountered in NMR of scalar-coupled two-spin systems (for example, 15N-1H groups in peptides), by employing two selective resonant fields applied to a pair of connected transitions with RF amplitudes much weaker than the sepn. of the parallel transitions (e.g., the scalar coupling const. JNH). The transfer benefits from differential line narrowing due to interference between the 15N chem. shift anisotropy and 15N-1H dipolar coupling, which is clearly obsd. in 15N-labeled human ubiquitin at 600 MHz. [on SciFinder (R)]