Withanolide C (WHC) inhibited the cell viabilities of breast cancer and normal breast cells differentially. Cell viabilities were determined by ATP assays. (A) ATP and MTS assays for determining cell viability after WHC treatment. For the ATP assay, breast cancer (SKBR3, MCF7 and MDA-MB-231) cells and breast normal (M10) cells were exposed to 0 (0.1% DMSO only), 0.25, 0.5, 0.75, and 1 μM of WHC for 48 h. For the MTS assay, breast cancer (SKBR3 and MCF7) cells and M10 cells were exposed to 0 (0.1% DMSO only) and 1 μM of WHC for 48 h. WHC also dissolved in the same concentration of DMSO. (B) N-acetylcysteine (NAC) pretreatment reversed WHC-induced ATP changes. Following pretreatment with NAC (10 mM for 1 h), cells were treated with the control and 1 μM of WHC for 48 h, i.e., NAC/WHC. (C) ATP assay for determining cell viability after cisplatin treatment for 48 h. Data are means ± SDs (n = 3). Results marked without overlapping letters show significant differences (P < 0.05 to 0.0001).