Used as a vital stain in biological assays to detect cellular respiration. It turns red upon reduction by metabolically active cells, forming formazan, which allows visualization of living tissue. Commonly applied in seed viability testing, where living embryo tissues stain red while dead tissues remain colorless. Also used in microbiology to assess bacterial activity and in plant physiology to evaluate root activity and tissue health. Its color change provides a simple, visual indicator of metabolic function.