The activity of nonsense-mediated mRNA decay (NMD) cascade in several mammalian cell lines was investigated. The analysis was conducted with the help of the recently developed genetically encoded fluorescent sensor [Pereverzev et al.]. This fluorescent NMD-reporter allows estimating the NMD activity in single live cells from the ratio of signals of the green and red fluorescent proteins. The following cell lines were analyzed: mouse colon carcinoma CT26, mouse Lewis lung carcinoma LLC, human T-cell leukemia Jurkat, and spontaneously immortalized human keratinocytes HaCaT. These cell lines demonstrated very different NMD activities. The NMD activity in the CT26 cells was low, while in the LLC cells - high (8.5-fold higher than in CT26). The Jurkat and HaCaT cells demonstrated high heterogeneity and included two subpopulations of cells with low and high NMD activities. In addition, high NMD activity was demonstrated for the primary culture of mouse embryonic hippocampal neurons.