'Salt and pepper' staining may be described as minute circular dark spots interspersed with conventional diffuse punctate staining. These dark spots appear to be pinpoint areas of negative stain that are slightly smaller than adjacent punctate stains. Size and number estimates were obtained using high magnification slit lamp freeze-frame video images. Dark spot size estimates of 20 to 40 microns were found, corresponding roughly to the dimension of surface epithelial cells. For a moderate example, 30 to 50 dark spots were observed per square millimeter. When this clinical presentation is compared with low magnification SEM micrographs of rabbit corneas exposed to chemical disinfecting agents, a possible etiology can be proposed. Corneas exposed to toxic agents may have numbers of cells that exhibit intracellular edema while others exfoliate. Exfoliation produces conventional punctate staining. The edematous, swollen cells rise above the plane of the adjacent corneal surface and appear as dark spots against the uniform fluorescence of the tear film due to localized tear thinning. Also, as high magnification SEM reveals, these cells typically lose normal surface microvilli that aid in mucin adherence. This altered surface morphology may facilitate pinpoint areas of tear break-up, or true 'negative stain'. These findings suggest a correlation between clinical biomicroscopy and SEM and that negative staining of this type is pertinent in evaluating corneal response to disinfectants and preservatives. Study funded by Allergan, Inc.