  The energy density of energetic protons, U_p, in several nearby starburst
nuclei (SBNs) has been directly deduced from gamma-ray measurements of the
radiative decay of neutral pions produced in interactions with ambient protons.
Lack of sufficient sensitivity and spatial resolution makes this direct
deduction unrealistic in the foreseeable future for even moderately distant
SBNs. A more viable indirect method for determining U_p in star-forming
galaxies is to use its theoretically based scaling to the energy density of
energetic electrons, U_e, which can be directly deduced from radio synchrotron
and possibly also nonthermal hard X-ray emission. In order to improve the
quantitative basis and diagnostic power of this leptonic method we reformulate
and clarify its main aspects. Doing so we obtain a basic expression for the
ratio U_p/U_e in terms of the proton and electron masses and the power-law
indices that characterize the particle spectral distributions in regions where
the total particle energy density is at equipartition with that of the mean
magnetic field. We also express the field strength and the particle energy
density in the equipartition region in terms of the region's size, mean gas
density, IR and radio fluxes, and distance from the observer, and determine
values of U_p in a sample of nine nearby and local SBNs.
