Antichick fibronectin antiserum, noncross-reactive to bovine fibronectin, was prepared to determine the production of fibronectin by cultured chick retinal pigment epithelial cells which were grown in the presence of fetal bovine serum. The typical fibrillary net pattern of fibronectin was observed by an indirect immunofluorescent technique when this specific antiserum reacted with cultured chick retinal pigment epithelial cells. Cultured chick retinal pigment epithelial cells were shown to produce fibronectin.