The aim of the present study was to investigate in detail the molecular mecha-nisms by which free fatty acids induce liver toxicity in liver cells. HepG2 and Huh7 human liver cell lines were exposed to varying concentrations of stea-rate (18:0), oleate (18:1), or mixtures of the two fatty acids, and the effects on cell proliferation, lipid droplet accumulation and induction of endoplasmic reticulum stress and apoptosis were evaluated.