Using a chemical evolution model we investigate the intriguing suggestion that there are populations of stars in some globular clusters ( e.g .
NGC 2808 , \omega Centauri ) with enhanced levels of helium ( Y \sim 0.28 to 0.40 ) compared to the majority of the population that presumably have a primordial helium abundance .
We assume that a previous generation of massive low-metallicity Asymptotic Giant Branch ( AGB ) stars has polluted the cluster gas via a slow stellar wind .
We use two independent sets of AGB yields computed from detailed models to follow the evolution of helium , carbon , nitrogen and oxygen in the cluster gas using a Salpeter initial mass function ( IMF ) and a number of top-heavy IMFs .
In no case were we able to fit the observational constraints , Y > 0.30 and C + N + O \approx constant .
Depending on the shape of the IMF and the yields , we either obtained Y \gtrsim 0.30 and large increases in C + N + O or Y < 0.30 and C + N + O \approx constant .
These results suggest that either AGB stars alone are not responsible for the large helium enrichment or that any dredge-up from this generation of stars was less than predicted by standard models .