  We give, in the framework of the bosonic string theory, simple prescriptions
for computing, at tree and one-loop levels, off-shell string amplitudes for
open and closed string massless states. In particular we obtain a tree
amplitude for three open strings that in the field theory limit coincides with
the three-gluon vertex in the usual covariant gauge and two-string one-loop
amplitudes satisfying the property of transversality.
