Abstract
If the b and c quarks mix with new heavy quarks of weak isospin I3=−1 and 0, respectively, then the Z→bb¯(cc¯) rate is necessarily greater (smaller) than that of the standard model. This may be the reason for the Rb excess and Rc deficit observed at CERN LEP. A possible consequence of this scenario is the prospective discovery of a new quark x with the dominant decay x→ch, then h→bb¯, where h is a neutral Higgs boson.