  Low temperature susceptibility and specific heat for single crystals of
Pr$_{1-x}$La$_x$Os$_4$Sb$_{12}$ (0 $\leq x$ $\leq 1$) are reported. La-doping
leaves CEF energies of Pr essentially unchanged. The average $T_c$ is only
weakly affected by the La-substitution and varies approximately linearly
between the end-compounds. The second superconducting transition disappears
between $x$=0.05 and 0.1. The discontinuity in $C/T$ at $T_c$, on the other
hand, is drastically reduced from about 1000 mJ/K$^2$mol for $x$=0 to 200
mJ/K$^2$mol for $x$=0.2. $\Delta C/T_c$ decreases further for alloys
corresponding to $x\geq 0.6$ to the value of a conventional superconductor
LaOs$_4$Sb$_{12}$. This behavior implies non-single impurity origin of the
heavy fermion state in PrOs$_4$Sb$_{12}$. We argue that critical quadrupolar
fluctuations are responsible for this heavy fermion state.
