The anti-inflammatory activity of onion juices prepared from sulfur-fertilized onions was investigated by measuring the secretion of proinflammatory cytokines from human monocytes cultured under hyperglycemic condition. Human monocytic (THP-1) cells were cultured under normoglycemic (NG, 5.5 mM glucose) or hyperglycemic (HG, 25 mM glucose) conditions, with or without onion juice. Without onion juice, cell viability decreased significantly in the HG state for 48 h, compared to that in the NG state. With onion juice ($50-150{mu}L$) treatment, the cell viability was not different from that under the NG condition, suggesting that onion juice prevented HG-induced monocytes cytotoxicity. While the HG condition in vitro significantly induced TNF-${alpha}$ release from THP-1 cells and its gene expression, onion juice ($50{mu}L$) significantly suppressed them. This indicates that onion juice inhibited HG-induced cytokine production in monocytes. These results suggest that onion juice from sulfur-fertilized onions can be used for the prevention of diabetes and related diseases.