The binding of chemical carcinogens to nuclear macromolecules, especially to DNA, is thought to be central to the initiation of carcinogenesis. Previous studies of the interactions of one such ultimate carcinogen, benzo[a]pyrene diol epoxide (BPDE-I) with nuclei, chromatin and purified DNA, demonstrated that although some BPDE-I-DNA interactions were altered in chromatin, covalent binding to chromatin DNA at saturating chromatin concentrations was quantitatively the same as binding to purified DNA. We have now extended these studies to include the basic subunit of chromatin, the nucleosomal core particle. Association constants for BPDE-I and a nonreactive analogue were determined by absorbance and fluorescence spectroscopy using either core particles or purified DNA and were found to be lower, by a factor of 30, for core particles. One of the major pathways of interaction of BPDE-I with DNA is the catalysis of BPDE-I hydrolysis by the exocyclic amino group of deoxyguanosine in native DNA. This detoxification reaction is inhibited about 30-fold in core particles compared with DNA, consistent with the hypothesis that intercalation is important in this catalytic reaction. In contrast to these findings, at DNA concentrations that allow maximal binding, similar amounts of BPDE-I are bound covalently to either free DNA or the DNA contained in core particles. This finding suggests that the interaction of DNA with histones to form the subunit structure of chromatin does not significantly protect DNA from damage by this ultimate carcinogen. The pattern of DNA adducts formed with core particle DNA shows a subtle shift toward the pattern seen with denatured DNA.