  A new mechanism for neutrinoless double beta ($\znbb$) decay based on
leptoquark exchange is discussed. Due to the specific helicity structure of the
effective four-fermion interaction this contribution is strongly enhanced
compared to the well-known mass mechanism of $\znbb$ decay. As a result the
corresponding leptoquark parameters are severely constrained from
non-observation of $\znbb$-decay. These constraints are more stringent than
those derived from other experiments.
