  We report a measurement of the angular distributions of Drell-Yan dimuons
produced using an 800 GeV/c proton beam on a hydrogen target. The polar and
azimuthal angular distribution parameters have been extracted over the
kinematic range $4.5 < m_{\mu \mu} < 15$ GeV/c$^2$ (excluding the $\Upsilon$
resonance region), $0 < p_T < 4 $ GeV/c, and $0 < x_F < 0.8$. The $p+p$ angular
distributions are similar to those of $p+d$, and both data sets are compared
with models which attribute the $\cos 2 \phi$ distribution either to the
presence of the transverse-momentum-dependent Boer-Mulders structure function
$h_1^\perp$ or to QCD effects. The data indicate the presence of both
mechanisms. The validity of the Lam-Tung relation in $p+p$ Drell-Yan is also
tested.
