  We revisit a method to obtain upper limits on the jet matter content
combining synchrotron self-absorption constraints and the large scale bubble
energy. We use both X-ray observations, which give limits on the jet power from
the energies and timescales of bubbles found in clusters of galaxies, and radio
observations, which give limits on the magnetic field in the jets. Combining
the two imposes constraints on the particle number density, and hence the jet
content. Out of a sample of clusters which have clear radio bubbles, there are
only two which have sufficient resolution in the radio images to give
significant constraints, under the assumption that the jets are fairly steady.
The results for M87 and Perseus indicate that the radio emitting region of the
jet is electron-positron dominated, assuming that the minimum of the electron
energy distribution, gamma_min~1.
