  We present chemical abundance analysis of a sample of 15 red giant branch
(RGB) stars of the Globular Cluster NGC~1851 distributed along the two RGBs of
the (v, v-y) CMD. We determined abundances for C+N+O, Na, $\alpha$, iron-peak,
and s-elements. We found that the two RGB populations significantly differ in
their light (N,O,Na) and s-element content. On the other hand, they do not show
any significant difference in their $\alpha$ and iron-peak element content.
More importantly, the two RGB populations do not show any significant
difference in their total C+N+O content. Our results do not support previous
hypotheses suggesting that the origin of the two RGBs and the two subgiant
branches of the cluster is related to a different content of either $\alpha$
(including Ca) or iron-peak elements, or C+N+O abundance, due to a second
generation polluted by SNeII.
