Cataracts were observed in female F344 rats who intravenously received methylnitrosourea (MNU), a potent carcinogen for multiple organs, in one dose of 50 mg/kg body weight. Induction of cataract 40 weeks after MNU treatment was 41% whereas no cataract was observed in control rats. The aggregation of beta H-crystallin fraction by MNU was studied in vitro. HPLC pattern of beta H-crystallin changed when lens protein was incubated for 24 hrs with MNU. HPLC patterns indicated that MNU induced high molecular weight aggregates of beta H-crystallin. This study conveys some indication about the direct interaction of MNU with lens protein in cataract formation.