  The Pr-rich end of the alloy series Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ has been
studied using muon spin rotation and relaxation. The end compound
PrOs$_4$Sb$_{12}$ is an unconventional heavy-fermion superconductor, which
exhibits a spontaneous magnetic field in the superconducting phase associated
with broken time-reversal symmetry. No spontaneous field is observed in the
Nd-doped alloys for x $>$ 0.05. The superfluid density is insensitive to Nd
concentration, and no Nd$^{3+}$ static magnetism is found down to the lowest
temperatures of measurement. Together with the slow suppression of the
superconducting transition temperature with Nd doping, these results suggest
anomalously weak coupling between Nd spins and conduction-band states.
