  The "semiquantum" key distribution protocol introduced by Zou et al. [Phys.
Rev. A Vol.79, 052312 (2009)] is examined. The protocol while using two-way
quantum communication requires only Bob to be fully quantum. We derive a
trade-off inequality between information gained by Eve and the disturbance
observed by legitimate users. It guarantees that Eve cannot obtain large
information if the disturbance is sufficiently small.
