  We present results of calculations of the total and jet photoproduction cross
sections at HERA and Fermilab energies$^1$. The calculations take into account
the high-energy QCD structure of the photon and are performed for different
photon structure functions. We discuss how recent measurements of the total
photoproduction cross section at HERA energies$^2$ can provide an important
information on the low $x$ behavior of the photon structure function and in a
more general context, to the nature of strong interactions. In addition, we
show that the photoproduction cross section measurements at Fermilab E683
energies could provide a firmer value for $p_{T}^{min}$, the lower bound on the
transverse momentum of outgoing jets, which signals the onset of hard
scattering. The extrapolation of our cross section to ultra-high energies, of
relevance to the cosmic ray physics, gives significant contribution to the
``conventional'' value, but cannot account for the anomalous muon content
observed in the cosmic ray air-showers associated with astrophysical point
sources$^4$.
