  We report results of zero-field muon spin relaxation experiments on the
filled-skutterudite superconductors~Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$, $x = 0$,
0.07, 0.1, and 0.2, to investigate the effect of Ce doping on broken
time-reversal symmetry (TRS) in the superconducting state. In these alloys
broken TRS is signaled by the onset of a spontaneous static local magnetic
field~$B_s$ below the superconducting transition temperature. We find that
$B_s$ decreases linearly with $x$ and $\to 0$ at $x \approx 0.4$, close to the
concentration above which superconductivity is no longer observed. The
(Pr,Ce)Pt$_4$Ge$_{12}$ and isostructural (Pr,La)Os$_4$Sb$_{12}$ alloy series
both exhibit superconductivity with broken TRS, and in both the decrease of
$B_s$ is proportional to the decrease of Pr concentration. This suggests that
Pr-Pr intersite interactions are responsible for the broken<EMAIL_ADDRESS>The two
alloy series differ in that the La-doped alloys are superconducting for all La
concentrations, suggesting that in (Pr,Ce)Pt$_4$Ge$_{12}$ pair-breaking by Ce
doping suppresses superconductivity. For all $x$ the dynamic muon spin
relaxation rate decreases somewhat in the superconducting state. This may be
due to Korringa relaxation by conduction electrons, which is reduced by the
opening of the superconducting energy gap.
