  We apply the method of topological quantization to obtain the bosonic string
topological spectrum propagating on a flat background. We define the classical
configuration of the system, and construct the corresponding principal fiber
bundle (pfb) that uniquely represents it. The topological spectrum is defined
through the characteristic class of the pfb. We find explicit expressions for
the topological spectrum for particular configurations of the bosonic strings
on a Minkowski background and show that they lead to a discretization of the
total energy of the system.
