The Philly mouse develops a hereditary cataract about 5 weeks after birth. Although the causative agent is not known, data suggest that there is a correlation between cataract formation and the selective absence of a 27 kilodalton (27K) beta-crystallin lens polypeptide. The ontogeny of the 27K beta-crystallin polypeptide was examined in normal mice in order to evaluate its role in normal development and determine what impact its absence may have on the Philly mouse lens. A monoclonal antibody was used with the PAP method to immunocytochemically localize the 27K polypeptide in lenses of normal mice during development. beta-Crystallins detected with polyclonal antisera were found in differentiated fiber cells throughout the lens. In contrast, the 27K beta-crystallin polypeptide detected with a specific monoclonal antibody was not found in the fiber cells of the inner part of the lens (nucleus), but was specifically localized in the fiber cells of the outer part of the lens called the cortex. The polypeptide was found only in elongating and differentiated fiber cells and not in mitotically active epithelial cells. Although a minor component of the 2-day-old lens, the 27K polypeptide comprised a large portion of the 16-day-old lens including the anterior and posterior poles. These data show that the 27K polypeptide is a minor component of the embryonic lens, but becomes a major contributor to the postnatal lens. The 27K beta-crystallin lens polypeptide is abundant in the fiber cells of the normal postnatal mouse lens. The absence of the 27K polypeptide in the Philly mouse may contribute to the observed failure of fiber cells to differentiate in the Philly mouse after birth or may be deleterious in some other manner to normal lens development. The selective absence of the 27K beta-crystallin polypeptide, a defect which precedes cataract formation in the Philly mouse, is intriguing since it suggests a relationship between this major lens polypeptide and lens clarity.