  We consider quadruples of positive integers $(a,b,m,n)$ with $a\leq b$ and
$m\leq n$ such that any proper edge-coloring of the complete bipartite graph
$K_{m,n}$ contains a rainbow $K_{a,b}$ subgraph. We show that any such
quadruple with $a\leq m$ and $n>(a^2-a+1)(b-1)$ satisfies this property. We
also show that the quadruple $(2,3,3,6)$ satisfies this property. We end with a
conjecture.
