This protocol describes the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay for assessing nanoparticle cytotoxicity in cultured cells. The method quantifies cell viability based on mitochondrial metabolic activity and provides IC₅₀ values for nanoparticle formulations. The protocol includes cell seeding, nanoparticle treatment, MTT incubation, and colorimetric measurement.