The distribution of basic fibroblast growth factor (bFGF) was studied immunohistochemically in fresh frozen sections of normal human tissues. Immunodetection was performed with a specific anti-bFGF mouse monoclonal antibody that was found to react with recombinant human bFGF in Western blot analysis, and to specifically neutralize the mitogenic activity of bFGF on bovine vascular endothelial cells. Expression of bFGF on normal human tissues was ubiquitously detected in the basement membranes of all size blood vessels, but was not found in epidermal or epithelial basement membranes of a variety of tissues tested. Intensity and patterns of localization in blood vessels was consistent in various tissues, but varied among different regions of the vascular bed. Whereas homogeneous and intense immunoreactivity were observed in large and intermediate size blood vessels, heterogeneity of expression was found in capillaries. The most intense immunoreactivity was observed in branching capillaries. Endothelial cell staining was heterogeneous and varied in different regions. Strong staining for bFGF was also found in cardiac muscle fibers, smooth muscle cells of mid-size blood vessels, the gut and the myometrium, in central nervous system neurons and cerebellar Purkinje cells, and on epithelial cells of the bronchi, colon, endometrium, and sweat gland ducts of the skin. The presence of bFGF in the extracellular compartment of a diverse variety of organs may play a role in angiogenesis. However, the function of bFGF in parenchymal cells remains to be determined.