Abstract We analyze the limits on deviations of the lepton and quark weak-couplings from their standard model values in a general class of models where the known fermions are allowed to mix with new heavy particles with exotic SU(2) × U(1) quantum number assignments (left-handed singlets or right-handed doublets). These mixings appear in many extensions of the electroweak theory such as models with mirror fermions, E 6 models, etc. Our results update previous analyses and improve considerably the existing bounds. As experimental constraints we use the new results on M Z, Γ Z, on the Z partial decay-widths and on the asymmetries measured at the Z-resonance, as well as updated results on the W-mass, on deep-inelastic ν-q and ν-e scattering and on atomic parity violation. Present constraints on lepton universality, unitarity of the quark mixing matrix and induced right-handed currents are also included. A global analysis of all these data leads to upper limits on the mixing factors s 2 ≡ sin 2 θ mix. When just one mixing is constrained at a time, we obtain for most of the fermions the light limits s 2 ≲ 0.002 ÷ 0.01 at 90% C.L. For u R, c R and ν τ the bounds are s 2 < 0.03, however if ν τ mixes with an ordinary heavy neutrino the constraint is s 2 ≲ 0.1 and a signal of non-zero mixing at 90% C.L. is found. For s R and b R we find the much weaker bounds s 2 ≲ 0.35. The constraints are weakened by a factor between 2 and 5 if accidental cancellations among different mixings are allowed to occur.