In this study we describe the tissue distribution of mouse CD40 using two monoclonal antibodies (mAb) against different epitopes of the molecule. In lymphoid tissues CD40 was expressed by B lymphocytes. Most B cells in typical B-cell compartments were CD40-positive, including germinal centre B cells. Interestingly, the two CD40 epitopes were differentially distributed on subpopulations of dendritic cells and epithelial cells. The 3/23 mAb, but not 3/3, recognized interdigitating dendritic cells (IDC) in lymph nodes, spleen and thymus. Langerhans cells were CD40 negative. In contrast, epithelial cells in the thymus and some other tissues (e.g. skin) were stained with the 3/3 mAb, but not with the 3/23 mAb. The expression of CD40 on dendritic cells and epithelial cells is in agreement with earlier findings in humans. Our data also demonstrate that different epitopes of CD40 are differentially expressed on dendritic cells and epithelial cells. This suggests the existence of different forms of CD40, that are expressed in a cell-type-specific fashion.