We investigated the effect of an antitransferrin receptor immunotoxin (454A12-rRA) on proliferating human and baboon lens epithelium in vitro. Human and baboon lens epithelial cells grown in modified TC-199 medium at 35 degrees Celsius in 7% CO2 were seeded in 24 well plates at a density of 17,500 cells/ml to 40,000 cells/ml. The cells were exposed to various concentrations of 454A12-rRA for seven days. The sensitivity of proliferating human lens epithelium to 454A12-rRA was dependent on the dose, with a 60% to 70% reduction in cell counts at immunotoxin concentrations of 100 ng/ml and above. The immunotoxin had no significant effect on baboon lens epithelium in vitro, which suggests that it is specific for human tissue. By preventing the proliferation of human lens epithelial cells, immunotoxin 454A12-rRA may be useful in the management of posterior capsule opacification after planned extracapsular cataract surgery.