  Consequences of relativistic causality for measurements of nonlocal
characteristics of composite quantum systems are investigated. It is proved
that verification measurements of entangled states necessarily erase local
information. A complete analysis of measurability of nondegenerate spin
operators of a system of two spin-1/2 particles is presented. It is shown that
measurability of certain projection operators which play an important role in
axiomatic quantum theory contradicts the causality principle.
