The Main Intrinsic Polypeptide (MIP) of the ocular lens fiber cell plasma membrane was immunocytochemically localized at the ultrastructural level on ultrathin frozen sections of rat lens, and on extracted, gradient-purified bovine lens membranes. The results indicate that both the junctional and non-junctional membrane domains of the cortical lens fiber cell are MIP immunoreactive. Frozen thin section immunocytochemistry of the lens epithelium and hepatocytes, also using anti-MIP antibodies, revealed that these cells, and their intercellular junctions, are not MIP-immunoreactive. From these findings we conclude that 1) MIP, a putative fiber cell junctional protein, is present throughout the plasma membrane of the lens fiber cell, and is not confined to the fiber cell junctional domain, 2) MIP is not a detectable component of the lens epithelial cell membrane, or its intercellular junctions, 3) MIP is not detectable in gap junctions of hepatocytes.