The pH of the conjunctival fluid was measured with a micro-glass-electrode in the inferior conjunctival fornix of 200 persons. Among 41 binocularly normal persons the pH was found to be 6.93 +/- 0.24 (mean +/- SD) of the primarily examined eye, independent of sex and age. The conjunctival fluid was significantly more acid in contact lens wearers (6.66 +/- 0.28, N = 26), but became normalized after removal of the lens (6.49 +/- 0.32----7.09 +/- 0.19, N = 8, P less than 0.001. The conjunctival fluid was significantly more alkaline in patients with lacrimal stenosis (7.13 +/- 0.28, N = 11), keratitis (7.14 +/- 0.28, N = 10), especially mycotic keratitis and postoperatively (7.15 +/- 0.22, N = 10, the first 24 h after corneal transplantation). There was, perhaps, a tendency towards alkalisation in eyes with keratoconjunctivitis sicca, whereas the pH was normal in the other pathological states, or the number of cases representing these may be too small.