The Jagiellonian Positron Emission Tomograph (J-PET) is a detector for tests of discrete symmetries as well as for biomedical imaging. The novelty of the system is based on usage of plastic scintillators for active detection material and trigger-less data acquisition system. The apparatus consists of 192 plastic scintillators read out
from both ends with vacuum tube photomultipliers. Positronium being an eigenstate of both the C and P operators is an unique probe to test the CP symmetry. This test performed at J-PET is based on determination of polarisation of photons from positronium annihilation. This allows exploration of a new class of discrete symmetry odd operator that was not investigated before.