Effects of metallic nickel particles on cell viability and apoptotic induction. JB6 cells were exposed to various concentrations of metallic nickel nano- or fine particles for 24 h. Cell viability was detected by MTT assay. Significantly less viability was observed in cells treated with nanoparticles compared to fine particles analyzed by R software (p < 0.05). Data shown are means ± S.E. of four independent assays (A). Apoptosis induced by metallic nickel nano- or fine particles was detected by YP staining (B, 10× magnification). Metallic nickel nanoparticles induced more apoptosis than fine particles at 0.5 and 5 μg/cm2 analyzed by Student's t-test (p < 0.05) indicated by * (C). Data shown are means ± S.E. of three independent assays.