A modified version of the LASER computer code is constructed by using Gear's method for isotopic burnup calculations. It is noted that the original version of the LASER code (as supplied recently by the Argonne Computer Code Center) errs in computation of 241Pu concentration due to a minor programming oversight. This error is corrected, and it is found that the corrected LASER and the modified LASER provide results that agree quite well. For the same calculations, the modified LASER requires only two-thirds the central processor unit time compared to the original (corrected) LASER (on an IBM 370/158). Also, the modified LASER has simpler and more general provisions for handling burnup calculations.