Summary: Antisera produced in the duck against pooled rabbit corneas exhibited cytotoxic effects on various rabbit cells grown in vitro. Corneal epithelium, endothelium and conjunctival epithelium were damaged by these antisera. The anti-cornea sera showed no significant reactions with rabbit corneal stromal fibroblasts. Conjunctival and spleen fibroblasts and heart “fibroblasts” were also completely unaffected. No effects on rabbit kidney epithelial cultures were observed. Antisera against rabbit lens and rabbit heart also showed cytotoxic effects against rabbit corneal epithelial cells. An apparent difference was found between fibroblasts grown from rabbit corneal stroma, conjunctiva and spleen, and the fibroblast-like cells grown from rabbit heart. The heart cells were profoundly damaged by anti-heart serum, while they were unaffected by anti-corneal and anti-lens sera. None of the sera had any effect on the fibroblasts grown from the other tissues.