  The low-field magnetic response of the non-centrosymmetric superconductor
YPtBi ($T_c =0.77$ K) is investigated. Ac-susceptibility and dc-magnetization
measurements provide solid evidence for bulk superconductivity with a volume
fraction of $\sim 70\%$. The lower critical field is surprisingly small:
$B_{c1} = 0.008$ mT $(T \rightarrow 0)$. Muon spin rotation experiments in a
transverse magnetic field of 0.01 T show a weak increase of the Gaussian
damping rate $\sigma_{TF}$ below $T_c$, which yields a London penetration depth
$\lambda = 1.6 \pm 0.2~ \mu$m. The zero-field Kubo-Toyabe relaxation rate
$\sigma_{KT}$ equals $0.129 \pm 0.004~\mu$s$^{-1}$ and does not show a
significant change below $T_c$. This puts an upper bound of 0.04 mT on the
spontaneous magnetic field associated with a possible odd-parity component in
the superconducting order parameter.
