Heavy Families: Masses and Mixings*

We use the infrared fixed point structure of the SU(3) x SU(2) x U(1) renormalization group equations to derive predictions for the masses and mixings of heavy families in perturbatively unifiable grand unified theories. We show that the sum of the squares of all quark (or lepton) masses must be less than (355 GeV)2. This implies that theories with NH heavy families must have at least one new quark and one new lepton of mass less than 250/a GeV. We -also find that the Cabibbo mixings and isospin splittings of heavy quarks tend to be small.