Natural substances offer interesting bioactivity patterns including antiproliferative, antioxidant or cytotoxic effects. However, the safety profile of many of them has not been extensively determined. In this study, the cytotoxic effect of Aeruginosin-865, resveratrol and capsaicin at different concentrations was tested on normal mouse cells (NIH/3T3) and tumour fibroblasts (WEHI-13VAR) as well as on liver- and kidney-derived cells from fallow deer. A lactate dehydrogenase cytotoxicity assay kit was used to measure cell death in response to treatment with the test substances. It was found that NIH/3T3 cells tolerated Aeruginosin-865 (10–200 M) and resveratrol (5–100 M) treatment without any cytotoxic effect, while capsaicin exerted a cytotoxic effect only at the highest tested concentration (200 M). Mouse fibrosarcoma cells were more sensitive to the cytotoxic effect of all three compounds where Aeruginosin-865 (100–200 M) and resveratrol (50–100 M) showed high-dose cytotoxicity and capsaicin showed low- and high-dose cytotoxicity (25 M and 200 M). The three tested compounds at the highest concentrations were found to be cytotoxic to both liver- and kidney-derived cells from fallow deer. Overall, the results indicate that the cytotoxic effects of the three tested natural substances on cells derived from fallow deer and mouse tumour fibroblasts differ significantly from those exerted on normal fibroblasts. The results demonstrate the potential of these natural compounds as therapeutic agents and pave the way for future in vivo toxicological investigations.