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 .