We evaluate in perturbative QCD, up to order alpha (4)(s) the mass of the B
-c. We use the so-called 1S mass in order to improve the convergence of the
perturbative series. Our result is E(B-c)(pert) = 6326(-9)(+29) MeV. Nonpe
rturbative effects are discussed. A comparison with potential models seems
to be consistent with nonperturbative contributions of the order - (40-100)
MeV.