After-cataract, or posterior lens capsule opacification, is an undesirable but common sequela to extracapsular cataract surgery. We are investigating biochemical means to prevent after-cataract formation, which can be applied at the time of the original surgery. Based on similar research efforts in cancer chemotherapy, we have prepared a conjugate of the antimetabolic agent methotrexate with an antibody specific for basement membrane collagen, the major protein in the lens capsule. The conjugate was evaluated using biochemical measurements, and retained both antimetabolic and antibody activities. When the conjugate was applied to bovine posterior capsules in vitro, or in vivo in rabbits, it was an effective inhibitor of lens epithelial cell outgrowth in cell culture.