Ascorbic acid is present as a primary antioxidant in plasma and within cells, protecting both cytosolic and membrane components of cells from oxidative damage. The effects of intracellular ascorbic acid on F-2-isoprostanes (biomarkers of oxidative stress) and monocyte chemoattractant protein-1 (market- of inflammatory responses) production in monocytic THP-1 cells were investigated under conditions of 2,2'-Azobis(2-methylpropionamidine) dihydrochloride (AAPH) induced oxidative stress. Cells cultured under normal conditions have extremely low ascorbate levels and the intracellular ascorbate can be augmented significantly by adding ascorbate to the culture medium. While AAPH treatment reduced cell viability, increased F-2-isoprostanes and MCP-1 production, the presence of intracellular ascorbic acid maintained high cell viability and attenuated both F-2-isoprostanes and MCP-1 production. Measurement of intracellular ascorbic acid and its oxidised products showed that intracellular ASC was oxidised to a significantly greater extent during AAPH treatment and may be utilised to protect the cells under conditions of oxidative stress. This study demonstrates the importance of intracellular ascorbate, which may be lacking under normal cell culture conditions, under conditions of increased oxidative stress. (c) 2006 Elsevier Inc. All rights reserved.