The immunogenical response to heteroimmunization and its effects on the lens were studied in 10 healthy young rabbits who received human lens antigen in Freund's adjuvant, according to a schedule. Half the immunized rabbits were treated with prednisolone (the equivalent of 10 mg/kg) daily. All five animals who did not receive prednisolone developed precipitating antibodies to human lens protein, whereas only one out of the five prednisolone-fed animals developed antibodies. Slitlamp examination of the lenses at 16 weeks and 32 weeks post-immunization revealed small cortical lens opacities in three of the animals who also showed lens antibodies in their sera. None of the antibody-negative animals showed any lens lesions (prednisolone treated).