  We summarize the top-quark mass measurements from the CDF and D0 experiments
at Fermilab. We combine published Run-I (1992-1996) measurements with the most
recent preliminary Run-II (2001-present) measurements using up to 2.8 fb^-1 of
data. Taking correlated uncertainties properly into account the resulting
preliminary world average mass of the top quark is M_t = 172.4 +/- 0.7 (stat.)
+/- 1.0 (syst.) GeV/c^2, assuming Gaussian systematic uncertainties. Adding in
quadrature yields a total uncertainty of 1.2 GeV/c^2, corresponding to a
relative precision of 0.7% on the top-quark mass.
