Expression of the immortalizing oncogene polyoma large T antigen (PyLT) in the lens of transgenic mice impairs fiber cell differentiation but does not induce hyperplasia or tumor development. To examine this phenotype further, we studied the expression of the lens-specific crystallin genes in normal mice and mice of the transgenic alpha PyLT1 lineage. Immunochemical analyses showed that the spatial pattern of expression of alpha, beta, and gamma crystallins was abnormal in lenses from alpha PyLT1 mice, and that abnormally differentiated lens cells expressed beta and gamma crystallins. We also found that the levels of expression of the crystallin proteins in the transgenic mice were reduced compared to those in normal mice. In vitro, epithelial lens cells from alpha PyLT1 mice expressed not only PyLT and alpha crystallins, but also beta and gamma crystallins, which occur specifically in differentiated cells. Yet, despite their nonproliferative nature in vivo and their expression of differentiated lens cell markers, lens cells from alpha PyLT1 mice proliferated indefinitely in vitro. The growth of the alpha PyLT1 lens cells in culture was inhibited by treatment with exogenous basic fibroblast growth factor, which is thought to be involved in growth and differentiation of lens cells in vivo. These results suggest that factors in the microenvironment of the eye may be important in the process of tumor formation in vivo.