  We derive the low-energy electroweak effective lagrangian for the case of
additional heavy, unmixed, sequential fermions. Present data still allow for
the presence of a new quark and/or lepton doublet with masses greater than
$M_Z/2$, provided that these multiplets are sufficiently degenerate. Deviations
of the effective lagrangian predictions from a full one-loop computation are
sizeable only for fermion masses close to the threshold $M_Z/2$. Some of the
constraints on new sequential fermions coming from accelerator results and
cosmological considerations are presented. We point out that the new fermions
can significantly affect the production and decay rate into $\gamma \gamma$ of
the intermediate Higgs at LHC.
