IGFBP-rP1/mac25 is a recently described member of the insulin-like growth factor binding protein (IGFBP) family. It has structural homology to the other members of the IGFBP family but has a lower affinity for insulin-like growth factors (IGFs). In previous studies using RNA blot hybridization, it was shown that the expression of IGFBP-rP1/mac25 was ubiquitous in normal human tissues. In this report we show by immunohistochemistry that the expression of IGFBP-rP1/mac25 is actually restricted to certain organs and specific cell types. We used an antibody raised against a decapeptide of the C-terminal part of the protein that recognizes a approximately 37-kD protein under reduced conditions. The immunohistochemistry performed on normal human tissues showed a ubiquitous intense staining of peripheral nerves and a variable degree of positive staining in smooth muscle cells, including those from blood vessel walls, gut, bladder, and prostate. Cilia from the respiratory system, epididymis, and fallopian tube showed intense immunoreactivity. Most endothelial cells showed some positivity, whereas fat cells, plasma cells, and lymphocytes were negative. There was specific expression limited to certain cell types in the kidney, adrenal gland, and skeletal muscle, indicating a possible specialized function of IGFBP-rP1/mac25 in these organs. We further noted an opposite pattern of staining in the lining epithelium of breast (typically positive) and prostate glands (largely negative). The specific localization of IGFBP-rP1/mac25 as described implies a function of the protein. However, its regulation within the IGF axis or a possible direct action of IGFBP-rP1/mac25 remains to be demonstrated.