Spin and Isospin in S-Matrix Theory

The Gunson-Olive technique for exploiting physical region poles and the unitarity equations to derive the analytic properties of S-matrix elements is applied to particles which belong to isospin multiplets and to particles with spin. In the former case it is shown that, in the framework of S-matrix theory, crossing symmetry leads to the requirement that particles and antiparticles transform according to complex conjugate representations; and it is possible to clarify the origin of the familiar isospin crossing matrices. In the case of particles with spin the construction of appropriate ``spinor'' basic states is discussed and the analytic properties of the corresponding spinor S-matrix elements or ``M-functions'' are derived.