  Using exact Lanczos diagonalizations we have shown that the pair-hopping
model for {\it negative} $W$ exhibits a phase transition into $\eta$-{\it
superconducting state}. The transition occurs at {\it any band filling} and the
critical value $W_{c}$ varies between $W_{c} \simeq -1.75t$ at half-filling to
$W_{c} \simeq -2t$ for two particles on the lattice.
