[en] We derive a sum rule that provides an expression for the top mass in terms of a special combination of quantities that might be measured in a near future at LEP. These include the ratio of the Z width into b quarks to the width into charged leptons, and the forward-backward asymmetry for b quark production on Z resonance. The sum rule frees the top contribution from other possible terms that would cause a redefinition of the ρ parameter. This remarkable property is valid both in the Standard Model and in a rather large variety of models of New Physics. Assuming a realistic overall accuracy, the presence of a top heavier than 150 GeV would not escape a clean experimental identification