An anti-Stokes emission in LaCl3:Pr3+ was found after excitation by a laser line resonant with the H-3(6)-->P-3(0) transition. The intensity of the blue emission was enhanced by three orders of magnitude when U3+ was added into the crystal. A mechanism for the up-conversion observed is discussed as well as the possibility of an avalanche process in Pr3+ triggered by U3+ ions.