Reflecting colours from the fatty layer of the precorneal film have been studied using mat filter (grease-proof paper, parchment paper, tracing paper) in front of the slit lamp mirror, maximally open light slit in a half-lit room, and magnification x 15. The palpebral fissure was narrowed until occurrence of red interference colour (2000 A). In 206 normal eyes the fatty layer was 102 +/- 3nm (+/- SEM), or about 0.1 micron, independent of age, sex and BUT (break up time). Maximum on awakening. Coefficient of variation 12.7 per cent. An increased fatty layer was noticed in cases of blepharitis (129 +/- 8 nm), in 91 per cent wearing hard contact lenses and 73 per cent wearing soft contact lenses. The fatty layer was likewise seen to be augmented in patients with acute infectious conjunctivitis (193 +/- 3 nm), chronic infectious conjunctivitis (164 +/- 7 nm), and in all states complicated by bacterial infection. The fatty layer is normal in allergic and chronic simple conjunctivitis. Silicone oil was found to effect reduction of the fatty layer.