A unified theory based on a homogeneous fiber bundle Q(M,G/H) is discussed in detail. In spite of the fact that the theory retains the full G-gauge invariance, the physical gauge group K is shown to be K=H*/(H∩H*), where H* is the centralizer of H in G. A principal fiber bundle P(M,G,H) is also constructed by introducing an additional left action H on P(M,G) that commutes with the right action G, and a unified theory based on P(M,G,H) is discussed. It is shown that the theory based on Q(M,G/H) is, in fact, the H projection of the Einstein–Hilbert action from P(M,G,H), with the identification Q(M,G/H)=P(M,G,H)/H. [ABSTRACT FROM AUTHOR]