We evaluated the light-harvesting FIT probes in RNA Fluorescent In Situ Hybridization (FISH) experiments. The cells were fixated, permeabilized and incubated with J3 at 37 °C for 4 hours. Analysis of the fluorescence micrographs (Fig. ) showed that 30 % of Jurkat cells fluoresced with an intensity greater than the half maximum of the measured single cell fluorescence values. This value (= 2,500) was reached by only 7 % of the CCRF-CEM cells (Fig. ). Setting the threshold at 60% of the maximum measured single cell fluorescence detects 14.3% of Jurkat cells. None of the 442 CCRF-CEM cells counted reached this fluorescence intensity (= 3.000). FISH with C14 resulted in a rather narrow distribution of fluorescence amongst Jurkat cells centered at a MFI = 25,200 (Fig. ). The CCRF-CEM cells exhibited a wider distribution with 9.1 % of cells showing fluorescence greater than 40.000. Thresholding at this fluorescence intensity (= 67 % of the largest single cell fluorescence obtained with C14) included only 1.7 % of Jurkat cells.