  We compute off-shell three- and four-tachyon amplitudes at tree level by
using a prescription based on the requirement of projective invariance. In
particular we show that the off-shell four-tachyon amplitude can be put in the
same form as the corresponding on-shell one, exhibiting therefore the same
analyticity properties. This is shown both for the bosonic and the fermionic
string. The result obtained in the latter case can be extended to the off-shell
four-tachyon amplitude in type 0 theory.
