Reporter proteins find increasing application in biomedical studies in vitro and in vivo. However, to correctly interpret the results based on their use, it is important to understand whether reporter protein production is modulated in model cells and in what conditions such modulation may occur. Reporter activity was studied in Mel IL melanoma cells transiently transfected with a pCpG vector-based plasmid construct expressing firefly luciferase. Luciferase expression quickly dropped during the first two culture passages, which were followed by a quasi-stable period, when luciferase expression relatively slightly decreased with time. Phases of maximal and minimal luciferase production, which corresponded to the exponential and stationary growth phases, respectively, were observed during batch culture. When the medium was changed, luciferase production was stimulated in the stationary, but not exponential, cell growth phase. Severe hypoxia (0.1% O₂) decreased the luciferase amount, suggesting substantial modulation of cell metabolism in total and luciferase production in particular. The targeted drug vemurafenib suppressed the luciferase production in Mel IL cells, whereas DMSO, which is often used as a drug solvent in experiments with cells, stimulated the luciferase production. Based on the results, it was hypothesized that modulation of reporter protein production in mammalian cells reflects the adaptation of intracellular metabolism to external conditions and may be a source of incorrect interpretations of experiments using reporter proteins.