Summary: Duck anti-adult rabbit lens sera demonstrated striking cytotoxicity against rabbit lens epithelium in tissue culture, whereas unimmunized duck serum was without effect. In addition, duck antisera to 26-day-old fetal rabbit lenses revealed similar cytotoxic effects. Duck antisera to rabbit sclera or cornea showed only minimal damaging effects on rabbit lens epithelium. These data confirm and extend earlier observations on the differences in specificity of cytotoxic antigens in various tissues from a single species. Homologous antisera prepared in the rabbit against pooled rabbit lenses, did not reveal cytotoxic effects on rabbit lens epithelium. Rabbit antisera against human, bovine, frog and marine fish (menhaden) similarly failed to damage rabbit lens epithelial cells, although all of the antisera contained numerous precipitating organ-specific antibodies active against rabbit soluble lens components. The data thus indicate that the antigens involved in organ specificity of the lens were not concerned in the cytotoxic reactions.