Due to the combination of bright fluorescence on the target and weak emission of the single strands, the light-harvesting FIT probes enabled the sensitive detection of RNA in challenging matrices such as pure cell lysate. The usefulness of the probes was further demonstrated in the flow cytometric detection of mRNA in living T cells. Live cell RNA detection is challenging because unbound probes cannot be washed away. Previously, molecular beacons were used for the flow cytometric analysis of cells expressing high levels of target mRNA. False positive signaling can occur. Currently, methods based on nanoparticles and branched hybridization are used in RNA flow cytometry, mainly for the analysis of fixed cells. We have shown that the comparably small light-harvesting FIT probes have the potential to distinguish one living T cell clone from another by targeting the CDR3 region of TCR mRNA. Both flow cytometry and RNA FISH of fixed cells suggest that a distinction is possible when focusing on the top 10-20 % of cells emitting bright fluorescence. Our work could lay the foundation for a sorting (and expansion) of T cells having a specific reactivity (e.g. against cancer cells). In this regard, the mRNA encoding for the TCR should be a suitable target. Moreover, the markedly enhanced brightness resulting from the incorporation of light-harvesting dyes into FIT probes is anticipated to be beneficial in a multitude of application scenarios Supporting Information: Details about experimental procedures, synthesis and additional characterization data is provided.