  Resistivity measurements were performed on Pr$_{1-x}$La$_x$Os$_4$Sb$_{12}$
single crystals at temperatures down to 20 mK and in fields up to 18 T. The
results for dilute-Pr samples ($x=0.3$ and 0.67) are consistent with model
calculations performed assuming a singlet crystalline-electric-field (CEF)
ground state. The residual resistivity of these crystals features a smeared
step centered around 9 T, the predicted crossing field for the lowest CEF
levels. The CEF contribution to the magnetoresistance has a
weaker-than-calculated dependence on the field direction, suggesting that
interactions omitted from the CEF model lead to avoided crossing in the
effective levels of the Pr$^{3+}$ ion. The dome-shaped magnetoresistance
observed for $x = 0$ and 0.05 cannot be reproduced by the CEF model, and likely
results from fluctuations in the field-induced antiferroquadrupolar phase.
