  Spectra with a high signal-to-noise ratio of 50 stars which are just below
the main sequence turnoff and are members of M13 are presented. They yield
indices for the strength of the CH and the ultraviolet CN band. There is no
evidence for a variation in the strength of either feature from star to star in
this intermediate-metallicity galactic globular cluster, and thus no evidence
for primordial variations in the abundance of C and N in M13. This supports the
hypothesis that abundance variations found among the light elements in the
evolved stars of M13 by Suntzeff (1981), and commonly seen on the giant and
subgiant branches of globular clusters of comparable metallicity, are due
primarily or entirely to mixing within a fraction of individual stars as they
evolve.
