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

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 TRS. 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.

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