In our culture system, lens epithelial cells differentiate into fibres under the influence of fibre differentiation factor (FDF) from neural retinas. FDF stimulates DNA synthesis and cell migration within 24 hours. After 2 days we detect increasedα-crystallin synthesis and, after 4 and 6 days,β-andγ-crystallin synthesis, respectively. In this study, a quantitative analysis of DNA levels using the DAPI method shows increases in DNA/explant up to 2 days of culture. This is followed by a substantial drop in DNA/explant by 10 days. The early rise in DNA levels corresponds with the period of cell division stimulated by FDF. The drop in DNA levels corresponds with a significant increase in the proportion of pyknotic nuclei in the explants. Electron microscopy shows pyknotic nuclei in differentiated fibre cells. Since nuclear pyknosis is a normal event during terminal fibre differentiationin vivo, this study confirms that our explant system faithfully reproduces events in fibre differentiation.