Abstract
Quantum consistency of open string theories is considered at the one-loop level, using a unified and consistent regularization of all divergences. Finiteness of all amplitudes with open string states is shown to be possible for any SO (n) or USp (n) gauge group. Two consequences of the nonunitarity of tachyonic theories are discussed; the presence of unphysical cuts in amplitudes and the nondecoupling of null states. If not for contributions of the open string tachyon, decoupling of all null open bosonic string states would be obtained for SO (2D/2) independently of the regularization, corresponding to vanishing tadpoles of unphysical closed string states.
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