The covalent binding of [3H]aflatoxin B1 (AF) to liver DNA was determined, 6 h after oral administration to male rabbits. A Covalent Binding Index, CBI (mumol AF/mol DNA-P)/(mmol AF/kg b.w.) = 8,500 was found. Pretreatment of rabbits with AF coupled to bovine serum albumin in Freund's adjuvant led to the production of AF-directed antibodies. Administration of [3H]AF to such immunized rabbits resulted in a CBI of only 2,500, i.e., the immunization provided a protection by a factor of more than 3. Although this is encouraging evidence for the potential of active immunization against genotoxic carcinogens, a number of points will have to be clarified, such as the time course for the DNA binding and the question of a possible shift to other target cells.