Helicity and canonical spin operators of the Poincare algebra

In the study of the irreducible unitary representations [m, s] of the Poincaré group, we define an ``helicity'' spin operator through a fundamental connection between canonical and helicity bases. This operator, in covariant notation, is simply related to the well-known Bargmann-Wigner operator and to the canonical spin operator. Its spatial components generate an SU(2)-algebra and coincide with the elements of the Z-spin algebra recently proposed on different grounds by Braathen-Foldy. The very simple arguments developed here establish in a natural way the uniqueness of this algebra when helicity representations are studied.