The CD40-CD40L (CD154) interaction plays a pivotal role in the effector mechanisms of allograft rejection. Blockade of the CD40/CD40L costimulatory pathway prevents the development of chronic allograft rejection in several animal transplant models. The relevance of in situ CD40 and CD40L expression in human liver allografts was assessed by immunohistochemistry during ductopenic chronic rejection (CR). In CR allograft specimens (n โซุโฌ 8), marked CD40L expression was detected on Kupffer cells (KCs) and sinusoidal macrophages with a unique centrilobular distribution (P F .001). The CD40Lุ KCs and macrophages were shown to be CD68ุ after immunohistochemical analysis of serial sections with anti-CD68 monoclonal antibody. Moderate stain-ing of vascular and sinusoidal endothelial cells and mononuclear infiltrates was observed in some CR cases. These findings were in contrast to the absence of CD40L expression in controls (n โซุโฌ 11) consisting of stable liver allograft and normal liver tissue specimens. Only occasional CD40 expression in some cases of CR and controls was observed. In CR, CD40L (CD154) expression is manifested on KCs and macrophages. The present novel data show another important cellular source of CD40L expression and suggest a potential role of KCs/macrophages and CD40/CD40L costimulatory interactions in the pathogenesis of chronic rejection ductopenic liver allograft.