There are compelling reasons to opt for primary human natural killer (NK) cells when validating Ca2+ indicators. 1.) NK cells exhibit a high degree of vulnerability to stressors such as indicator loading or light exposure. 2.) The lack of research on NK Ca2+ signaling underscores the necessity for developing reliable assays. 3.) The increased utilization of NK cell therapies necessitates a more profound comprehension of Ca2+ dependent signal transduction. Consequently, an assay was developed to monitor cytosolic Ca2+ signals in individual NK cells simultaneously with their cytotoxic function against cancer cells. We used this assay to assess the suitability of fura-2, fura-PE3, fura-8, fura-10 or fura-red for quantifying Ca2+ signals in NK cells without compromising their cytotoxic function. In contrast to the widely used fura-2, its red-shifted derivative fura-10 did not interfere with NK cytotoxicity over several hours. It exhibited a superior signal-to-noise ratio and good dynamic range, accompanied by minimal bleaching or leakage. Fura-8 and fura-red also preserved NK cell cytotoxicity, but had other disadvantages compared to fura-10. We successfully used fura-10 to report Ca2+ signals in NK cells from blood donors and patients diagnosed with lymphoma and leukemia over several hours at 37 °C during apoptotic or necrotic killing of different cancer cells (K562, THP1, OCI-AML2, and TMD8). Additionally, we show that fura-10 is well suited to report Ca2+ signals in intact murine pancreatic islets, another stress-sensitive cell preparation. Consequently, fura-10 is an optimal choice for measuring Ca²⁺ in primary human NK cells and other primary cell preparations.