  We report on the abundances of helium, carbon, nitrogen and oxygen in a
larger sample of Galactic massive stars of ~7-20 M_sun near the main sequence,
composed of apparently normal objects, pulsators of beta-Cephei- and SPB-type,
and magnetic stars. High-quality spectra are homogeneously analysed using
sophisticated non-LTE line-formation and comprehensive analysis strategies. All
the stars follow a previously established tight trend in the N/C-N/O ratio and
show normal helium abundances, tracing the nuclear path of the CNO-cycles
quantitatively. A correlation of the strength of the mixing signature with the
presence of magnetic fields is found. In conjunction with low rotation
velocities this implies that magnetic breaking is highly efficient for the
spin-down of some massive stars. We suggest several objects for follow-up
spectropolarimetry, as the mixing signature indicates a possible magnetic
nature of these stars.
