Benzo[a]pyrene (BP) administered to female outbred ICR/Ha mice by oral intubation induced neoplasia in the forestomach after 30 weeks in more than 90% of treated animals. However, no tumors occurred at that site if 1% disulfiram was added to the experimental diet. This inhibition of tumor formation was paralleled by a large inhibition of macromolecular binding of [3H]BP and [14C]BP to RNA and protein in the forestomach. The inhibitory effect of specific binding was strongest in the RNA fraction isolated at 8,000 X g for 20 minutes (6 times that of the control) and weakest in the DNA fraction (no significant difference). The highest specific binding was measured in the RNA fraction isolated at 32,000 X g for 120 minutes and the lowest binding in the DNA fraction. The relative distribution of the BP binding was such that greater than 90% of the BP was bound to protein and less than 1% was bound to DNA. Of the total inhibition of BP binding to all the macromolecular species studied, that to protein constituted the largest component (95%). In contrast to the forestomach, no inhibitory effect of BP binding to DNA, RNA, or protein by disulfiram was found in the liver, which remained free of cancer under the experimental conditions employed.