D2-40 is an antibody that reliably detects human lymphatic endothelial cells. Because little is known about the extent to which other cells are stained, we have investigated this question in various tissues. Normal, reactive and neoplastic lymphatics (including lymphangiomas and Stewart-Treves syndrome) were stained by D2-40. Published findings indicate that some angiosarcomas and Kaposi are positive. Staining was also found in the following non-endothelial cells: mesothelial cells, follicular dendritic reticulum cells, Cajal cells, a few epithelial cells in the dermis, myoepithelial cells (e. g. breast), and basal cells (e. g. prostata), various mesenchymal cells. The following tumours were reactive: myofibroblastic tumours, gastrointestinal stromal tumour, mesothelioma, adenomatoid tumour, traumatic neuroma, seminoma, seminoma in situ and other testicular tumours, pleomorphic adenoma (one of two tumours positive), and follicular dendritic reticulum cell sarcoma. Despite this relatively broad spectrum of reactivity, we consider D2-40 to be the best commercially available antibody for the delineation of normal, reactive and neoplastic lymphatic endothelial cells.