The transport properties of dilute and dense CexLa1−xAl3 have been measured between 1.2 and 300 K. For x = 0.99, the drop of the resistivity at low temperature, usually ascribed to a coherence effect, appears insensitive to small lattice disorder. For the whole range of Ce concentration, the thermopower S(T) has a positive maximum related to the high Kondo temperature. At lower temperature, S(T) presents an additional negative peak due to spin interactions in dense systems (x ≤ 1) and to crystal field in dilute alloys.