  In a direct open string approach we analyze scattering of massless states on
a stack of D3-branes. First we construct vertex operators on the D-branes. The
4+6 splitting for the fermionic part is made possible by inserting
appropriately defined projection operators. With the vertex operators
constructed we compute various tree amplitudes. The results are then compared
with the corresponding field theory computations of the $\N=4$ SYM with
$\a'$-corrections: agreements are found. We comment on applications to AdS/CFT.
