It is shown that selective single-transition cross-polarization (ST-CP) may be achieved in four-level systems such as encountered in NMR (nuclear magnetic resonance) of scalar-coupled two-spin systems (for example, N-15-H-1 groups in peptides), by employing two selective resonant fields applied to a pair of connected transitions with RF amplitudes much weaker than the separation of the parallel transitions (e.g., the scalar coupling constant J(NH)). The transfer benefits from differential line narrowing due to interference between the N-15 chemical shift anisotropy and N-15-H-1 dipolar coupling, which is clearly observed in N-15-labeled human ubiquitin at 600 MHz.