Microglia are thought to produce a cytotoxic cytokine, tumor necrosis factor alpha (TNF alpha), in the pathological brain. Thus, the mechanism that suppresses the induction of TNF alpha in microglia is of interest. We found that lipopolysaccharide (LPS)-induced TNF alpha is strongly suppressed by pretreatment with specific protein kinase C (PKC) inhibitors, Gö6976 and bisindolylmaleimide, suggesting that PKC alpha plays a role in the signaling cascade of TNF alpha induction. Therefore, the association of PKC alpha with TNF alpha induction was investigated in PKC alpha-deficient microglia prepared by pretreatment with phorbol-12-myristate-13-acetate (PMA). PMA-pretreated microglia strongly decreased the TNF alpha induction in the absence of PKC alpha when activated with LPS. The lack of PKC alpha in the PMA-treated microglia was confirmed by determining conventional PKC activity. Taken together, the present study demonstrated that PKC alpha is a requisite PKC isoform in the signaling cascade of TNF alpha induction in LPS-stimulated microglia.