  We present the results from a calculation of the full electroweak two-loop
fermionic contributions to the effective leptonic mixing angle of the $Z$
boson, $\sin^2\theta_{eff}$, in the Standard Model. On-shell renormalization
and analytic calculations are performed for the three-point vertex functions at
zero external momenta, whereas irreducible three-point integrals for
non-vanishing external momenta are evaluated semi-analytically applying two
different methods. Comparisons with a previous calculation show complete
agreement.
