The cellular distribution of a highly antigenic fibroblast glycoprotein, identified previously as aminopeptidase M, was studied in vitro by immunofluorescence and immuno-electron microscopy. The antigen was found to be localized at the surface of live and fixed fibroblasts in suspension and in cell layer; clustering or patching on live cells could be observed. Immunofluorescence after permeabilization of the cells with acetone showed distribution of the antigen in cytoplasmic granules. The tissue distribution of this antigen was examined by immunofluorescence on frozen sections of various human organs. In addition to fibroblasts, renal tubules, liver, pancreas and gut were found to react selectively with the antibody preparation. Moreover, a specific localization of the reaction product was observed. In the kidney the epithelium and brush border of the proximal convoluted tubules were stained. In the liver, the bile canaliculi reacted; in the pancreas the acinar cells and in the gut the brush border of the mucosal cells of the villi and crypts were stained. The same cells did not stain with an antiserum prepared in a similar fashion against another fibroblast component, or with preimmune IgG. In most sections of the selectively stained tissues, a predominant localization at the apical pole of the cells was observed. The localization of the antigen in different tissues corresponds with the distribution of aminopeptidase M, thereby confirming its identity and the antigenic cross-reaction between the fibroblast and tissue enzyme.