We investigate the hypodiesis that the parsec-scale jets observed in extragalactic objects consist of an electron-positron (e+/-) pair plasma outflowing from the compact, X-ray emitting region of the source. Due to annihilation in the innermost regions, there is an upper limit on the number of e+/- pairs that can reach the parsec-scale jet. On the other hand, there is a lower limit on the density of particles in the jet: their density must be at least equal to that needed to emit the synchrotron radiation we observe. We derive the latter limit by assuming a power-law electron distribution, with a low-energy cut-off. By comparing these two limits, we find that the constituents of parsec-scale jets may be e+/- pairs produced in the innermost regions only if their minimum energy is greater than approximately 50 MeV. Remarkably, the same limit on the minimum particle energy is found in the case of jets consisting of electrons and protons in order that Faraday depolarization is avoided.
