The intracellular pH of a normal lens is 6.8 in the cortex and remains unchanged during culture in media buffered at pH 7.2. Incubation of rabbit lenses in calcium enriched media, either at 24°C or 37°C, results in lens opacification provided that the lens pH remains slightly acidic. Opacities are prevented in cultured lenses with an alkaline interior (pH 7.1–7.3) despite the accumulation of calcium (1.3 mM). The mechanism by which an intracellular pH shift from 6.8 to 7.1 prevents opacification in the presence of excess calcium is not known, but does not appear to depend upon the total level of bound calcium. This study provides the first data that opacification caused by calcium is associated with lens pH.