Scanning electron microscopy (SEM) of mouse cornea and conjunctiva fixed with picric acid-paraformaldehyde-glutaraldehyde (PA-P-G) mixture revealed a thin layer of amorphous material covering the microvilli of the corneal surface cells. At the transmission electron microscopic (TEM) level, this layer of material stained positively with dialyzed iron, alcian blue and cationized ferritin, all of which are markers for anionic sulfate or carboxyl groups. The corneal surface was negative for high iron diamine, which specifically stains sulfate groups. These results indicate that the murine ocular surface is rich in carboxyl groups. Treatment with neuraminidase prior to fixation significantly reduced (P less than 0.005) cationic ferritin binding, suggesting that most of the carboxyl groups at the ocular surface are associated with sialic acid residues. The corneal surface also stained positively at the TEM level when a periodic acid-thiocarbohydrazide-silver protein sequence (PA-T-SP) was applied. This result indicated the presence of periodic acid-Schiff (PAS)-positive glycoprotein and glycolipid at the ocular surface.