  We show that for any nonzero class $A$ in $H_2(X; \mathbb{Z}_2)$ in a
rational 4-manifold $X$, $A$ is represented by a nonorientable embedded
Lagrangian surface L (for some symplectic structure) if and only if $P(A)\equiv
(L) (mod\ 4)$; where $P(A)$ denotes the mod 4 valued Pontrjagin square of $A$.
