A new method has been developed for identifying blood vessel capillaries and distinguishing them from lymphatic capillaries. Highly purified antibodies to two ubiquitous components of basement membrane - Type IV collagen and laminin - were applied to fresh-frozen and formalin-fixed tissue sections rich in blood vessel capillaries (granulation tissue), rich in lymphatic channels (small intestine and mesentery), and rich in both (skin, lung, and breast). Staining patterns were evaluated using standard immunofluorescence and immunoperoxidase techniques. Blood vessel capillaries contained intact basement membranes with linear staining for both Type IV collagen and laminin. Lymphatic capillaries, in contrast, uniformly lacked immunoreactivity with antibodies to these basement membrane components. These lymphatic capillaries, which were indistinguishable from blood vessel capillaries in both size (cross-sectional area) and shape (vessel eccentricity), were confirmed as lymphatics by dye injection studies. Our immunohistological technique detected 7-10 times more blood vessels than hematoxylin and eosin staining. Both benign and malignant tumors of blood vessel endothelium including two capillary hemangiomas and two angiosarcomas demonstrated extracellular linear staining for both Type IV collagen and laminin, whereas two lymphangiomas and two lymphangiectasis were negative. This method then provides a sensitive technique for detecting blood vessel capillaries in tissue sections and a reliable means of distinguishing blood vessel capillaries from lymphatic capillaries. This method may also aid in the identification of tumors whose origin is blood vessel endothelium.