The development of lens structures in transgenic mice (lnl mice) which carry the diphtheria toxin A chain-coding sequence under the control of the alpha-crystallin promoter is examined here in detail. The initial stages of lens development during embryonic days 10.5 to 12.5 (E10.5-E12.5), including the invagination of the surface ectoderm to form a lens vesicle, closure of the vesicle to form a lens cup, and initial appearance of the lens itself, appeared identical in histologic analyses of lnl mice and genotypically wild-type littermate controls. However, by E12.5, cells in the central posterior lens of developing lnl mice appeared to be vacuolated and undergoing necrosis. This necrosis was quite prominent at E14.5 and the overall lens size was significantly reduced. The lenses of lnl mice continued to be present but were significantly smaller throughout embryonic development. The cells of these lenses were capable of undergoing biochemical differentiation, reacting with antibodies to both alpha- and beta-/gamma-crystallin. alpha-Crystallin expression was initiated at the appropriate time (E10.5) and maintained in most cells of lnl lenses. The expression of beta-/gamma-crystallins was surprising as these crystallins are expressed later in lens development after normal expression of alpha-crystallin and after the anticipated time of expression of the diphtheria toxin transgene. Despite extensive necrosis and cell death, lens structures persisted in lnl mice and disappeared only in the early postnatal period between days 3 and 6. Throughout the perinatal period, the remaining lens cells expressed both alpha- and beta-/gamma-crystallins. Prenatal development of the retina and ciliary body was relatively normal although the eye was significantly reduced in overall size. Some additional developmental defects were noted including persistent hyaloid artery and thickened cornea. In the perinatal period the rapidly expanding retina filled the entire eye leaving essentially no anterior or posterior chamber. These results clearly indicate that lens cells which are the target of diphtheria toxin-mediated cell ablation techniques persist for a significant time during development and thus place limitations on the interpretations of results obtained using this technique.