  We analyze quartic gauge-invariant interactions of massless higher spin
fields by using vertex operators constructed in our previous works and
computing their four-point amplitudes in superstring theory. The kinematic part
of the quartic interactions of the higher spins is determined by the matter
structure of their vertex operators;the non-locality of the interactions is the
consequence of specific ghost structure of these operators. We compute
explicitly the four-point amplitude describing the complete gauge-invariant
$1-1-3-3$ quartic interaction (two massless spin 3 particles interacting with
two photons) and comment on more general $1-1-s-s$ cases, particularly pointing
out the structure of $1-1-5-5$ coupling.
