  The process of teleportation of a completely unknown single-photon
relativistic state is considered. Analysis of the relativistic case reveals
that the teleportation as it is understood in the non-relativistic quantum
mechanics is impossible if no {\it a priori} information on the state to be
teleported is available. It is only possible to speak of the amplitude of the
propagation of the field (taking into account the measurement procedure) since
the existence of a common vacuum state together with the microcausality
principle (the field operators commutation relations) make the concept of the
propagation amplitude for the individual subsystems physically meaningless.
When partial {\it a priori} information is available (for example, only the
polarization state of the photon is unknown while its spatial state is
specified beforehand), the teleportation does become possible in the
relativistic case. In that case the {\it a priori} information can be used to
``label'' the identical particles to make them effectively distinguishable.
