AbstractThe effect if thalidomide on tumor necrosis factor alpha (TNF-α) produced in vitro by lipopolysaccharide (LPS) stimulated human cells was investigated. In cultures of LPS stimulated human mononuclear cells enriched for adherent cells and in cultures of LPS stimulated human monocytes of the cell line-THP-1, thalidomide ehanced the synthesis of TNF-α. When cultures of un-fractionated peripheral blood mononuclear cells were stimulated with LPS, thalidomide decreased the synthesis of TNF-α. Depending on the type of cells stimulated with LPS in vitro, thalidomide, at concentrations achieved in vivo, can either enhance or suppress the synthesis of TNF-α