Rabbits immunized against lens extracts from various other (xenogeneic) species reacted well with the immunizing lens extracts by immunological assays measuring either humoral or cellular immune responses. A dissociation was observed, however, between the humoral and cellular immune responses when lenses from rabbit (allogeneic) or from nonimmunizing species were tested. Antibodies from these rabbits cross-reacted with lenses from rabbit or all other tested species, with levels similar to those obtained with the immunizing lens extracts. The antibodies were determined by both serological tests and the Arthus skin reaction. On the other hand, these rabbits failed to cross-react with rabbit or nonimmunizing animal lens extracts by the cell-mediated delayed skin reaction, and their lymphocytes reacted poorly in culture with the nonimmunizing lenses. A similar dissociation between the humoral and cellular responses was found in rabbits immunized against allogeneic lens. Most of these rabbits produced moderate levels of lens antibodies demonstrated by the serological tests and the Arthus skin reaction but failed to exhibit delayed-type hypersensitivity, and their lymphocyte cultures reacted poorly or not at all to the rabbit or other lenses. The results are discussed in view of their relevance to the hypotheses concerning the pathogenesis of phacogenic uveitis.