During long-term culture of bovine lens epithelial cells, cell elongation and structures resembling lens fibers can be observed. After contact inhibition, large cell aggregates are formed. Within these aggregates cells are kept alive for more than 12 months. From the behavior of biochemical markers characteristic of fiber formation in vivo, it must be concluded that neither the structures morphologically resembling lens fibers nor the aggregates, represent products of a true fiber formation in vitro. The presence of TPO, an enzyme catalyzing thiol-disulfide exchanges, is demonstrated in lens cells for the first time.