Intense fluorescent staining of cells in the glomerular tuft has been demonstrated in a hypoxic dog kidney with an antibody to erythropoietin (ESF) using the indirect fluorescent antibody technique. The fluorescent staining in the glomeruli was blocked by a highly purified preparation of ESF. This fluorescence appears to be localized in the visceral epithelial cells of the giomerulus. Kidney sections of a normal dog and a dog injected with hypoxic dog plasma ESF to maintain a prolonged elevation in plasma ESF titers failed to show positive fluorescent staining following treatment with the antibody to ESF. The present studies suggest that the epithelial cells of the glomerular tuft are the site of ESF production.