  We report on a simultaneous NuSTAR and Swift observation of the neutron star
low-mass X-ray binary 4U 1728-34. We identified and removed four Type I X-ray
bursts during the observation in order to study the persistent emission. The
continuum spectrum is hard and well described by a black body with $kT=$ 1.5
keV and a cutoff power law with $\Gamma=$ 1.5 and a cutoff temperature of 25
keV. Residuals between 6 and 8 keV provide strong evidence of a broad Fe
K$\alpha$ line. By modeling the spectrum with a relativistically blurred
reflection model, we find an upper limit for the inner disk radius of $R_{\rm
in}\leq2 R_{\rm ISCO}$. Consequently we find that $R_{\rm NS}\leq23$ km,
assuming $M=1.4{\mbox{$\rm\,M_{\mathord\odot}$}}$ and $a=0.15$. We also find an
upper limit on the magnetic field of $B\leq2\times10^8$ G.
