In explant cultures, lens epithelial cells grown in unsupplemented medium retain a morphology and packing arrangement similar to that found in the lens in vivo. In this culture system the epithelial cells can be induced to differentiate into fibres by the addition of retina conditioned medium (RCM). RCM also stimulates cell division. In order to trace the fibre cell lineage and to examine the relationship between cell division and fibre differentiation, single epithelial cells in explants from neonatal rat lenses were labelled with fluorescein-isothiocyanate-dextran. Explants were sub-divided into nine squares and one cell per square was injected with fluorescent label via a microcapillary. By marking the positions of labelled cells at 24-hr intervals for 6 days it was shown that most of the epithelial cells moved laterally within the explant. On average, cells in control explants moved about 20 microns day-1. Cells did not move in any particular direction within the explants and often changed direction. RCM stimulated a dramatic increase in migratory activity. There was about a four-fold increase in migratory activity in the first 24-hr interval, then, even with continued exposure to RCM, this activity quickly dropped over the next 2 days to the same levels as found in controls. As in controls, the cells moved in no particular direction and often changed direction. The observation that RCM stimulates cell migration in this explant system raises the possibility of an important role for active cell migration in the lens in situ. After 6 days culture the dimensions of labelled cells were measured using an image analyser. The areas of cells in controls fell within a narrow range from about 60- to 200 microns2. In contrast, explants grown in RCM had a wide range of cell areas from about 120- to 1500 microns2 and a large proportion of the cells showed some degree of elongation. In explants grown in RCM, 27.3% of labelled cells divided and half of these divisions were during the first 24 hr of culture. Overall there were about 9% more divisions recorded in RCM-treated than in control explants. An analysis of sizes of cells after 6 days of culture showed that whether or not cells divided after addition of RCM they showed very similar frequencies of cell sizes. Therefore, proliferating cells in the explants appear to be as capable of elongating and differentiating into fibres as the non-proliferating cells.(ABSTRACT TRUNCATED AT 400 WORDS)