Figure 1. Blue light differentially altered survival and growth of cells. Succinate dehydrogenase (SDH) activity (estimating cell survival and growth, MTT method, expressed as optical density [OD] of solubilized formazan @ 560 nm) was measured at 24-hour intervals for 72 hrs after exposure to QTH (2 min, 60 J/cm²) and PAC (30 sec, 60 J/cm²) light sources (Table). Cells were fibroblastic (Balb/c 3T3-mouse lung, HGF-human gingival, WI-38-human lung) or epithelial (MCF-7-human breast carcinoma, OSC-2-human oral squamous cell carcinoma, NHEK-normal human foreskin) in origin. Cells were plated at 12,500 cells/cm² 24 hrs prior to light exposure at time zero. Both suppression (Balb/c, OSC-2, MCF-7) and stimulation (WI-38, NHEK) of cell growth were observed. Error bars indicate standard deviations, and asterisks (*) indicate SDH activity statistically different from no-light control within each time period (ANOVA, Tukey, = 0.05, n = 3).