5A11/Basigin is an immunoglobulin-like glycoprotein expressed on the surface of Müller cells, the apical and basal surfaces of the retinal pigmented epithelium, and photoreceptor cell bodies and their inner segments. Disruption of the 5A11/Basigin gene in the mouse results in photoreceptor degeneration and a corresponding decrease in electroretinogram amplitudes in mature mice. The purpose of this study was to examine the electrophysiology of the 5A11/Basigin null mouse retina at earlier ages than previously examined. Although the architecture of the 5A11/Basigin null mouse retina appears normal, the ERG amplitudes are severely depressed at eye opening, indicating failure in retinal maturation.