  Measurements of the upper critical field $H_{c2}$ near $T_c$ of Pr$_{1-
x}$La$_{x}$Os$_{4}$Sb$_{12}$ were performed by specific heat. A positive
curvature in $H_{c2}$ versus $T$ was observed in samples and concentrations
exhibiting two superconducting transitions. These results argue against this
curvature being due to two-band superconductivity. The critical field slope -
$dH_{c2}/dT$ suggests the existence of a crossover concentration $x_{cr}
\approx 0.25$, below which there is a rapid suppression of effective electron
mass with La-alloying. This crossover concentration was previously detected in
the measurement of the discontinuity of $C/T$ at $T_c$.
