Quantization of Non-Critical Bosonic Open String Theory
  The theory of relativistic strings is considered in frames of Hamiltonian formalism and Dirac's quantization procedure. A special gauge fixing condition is formulated, related with the world sheet of the string in Lorentz-invariant way. As a result, a new set of Lorentz-invariant canonical variables is constructed, in which a consistent quantization of bosonic open string theory could be done in Minkowski space-time of dimension $d=4$. The obtained quantum theory possesses spin-mass spectrum with Regge-like behavior. 
Igor Nikitin