CD14 has been reported to be the lipopolysaccharide (LPS)-LPS binding protein receptor. The effects of interferon-gamma (IFN-gamma) on CD14 expression have not been clearly established. The purpose of this investigation was to examine the effects of IFN-gamma alone and IFN-gamma followed by bacterial LPS on CD14 expression. Human peripheral blood monocytes were isolated by counterflow centrifugal elutriation (CCE). Monocytes were cultured for 48 h with IFN-gamma alone or for 24 h with IFN-gamma followed by LPS for a second 24 h. IFN-gamma alone caused a down-regulation of CD14 expression, as assessed by flow cytometry, relative to CD14 expression in untreated monocytes. In addition, CD14 expression was even more significantly down-regulated after IFN-gamma pretreatment followed by either Prevotella intermedia or Salmonella typhimurium LPS. Likewise, the percentage of CD14+ monocytes decreased after IFN-gamma alone and even more dramatically after IFN-gamma treatment followed by either LPS. This study clearly demonstrated that IFN-gamma down-regulates CD14 expression and that LPS following IFN-gamma pretreatment potentiates this effect.