Abstract: Use of a luciferase that has a mutation of at least one amino acid selected from the group consisting of positions 14, 35, 182, 232 and 465, where the numbering is according to the sequence of the luciferase from P. pyralis (SEQ ID NO:1) in a method that is performed at a pH below the optimal pH for the wild-type luciferase during at least part of the time period over which bioluminescence measurements are taken, wherein the specific activity of the mutant luciferase is higher than the specific activity of wild-type at the pH at which the method is carried out.