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 time-scales 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) similar to 1.