Let X be a variety and $E$ an ample vector bundle on $X$. Let $G=G(r+1,E)$ be the Grassmann bundle over $X$ whose fiber over $x\in X$ is the Grassmannian of the $r+1$-dimensional subspaces of $E_x$. Let $U$ denote the universal subbundle on $G$. Under which hypothesis is $U$ ample on $G$?