Methods and compositions are described for assaying luciferase bioluminescence in vitro and in vivo cells. The compositions provide at least one of enhanced stability of signal or magnitude of signal by varying the composition of the buffer. One or more of the following parameters have been varied: the presence or absence of EDTA, the concentration of NaCI, the concentration of coelenterazine, the evaluation of ionic and non-ionic detergent, the amount of detergent, how the detergent has been added and the time over which the signal has been recorded. Also disclosed are dual reporter systems.