Rat-liver nuclei were incubated with microsomal fraction, NADPH and [3H]benzo[a]pyrene ([3H]BP). From the incubated nuclei, H1 histone was extracted and purified. The purified histone was acid-hydrolyzed; the hydrolysate was filtered on a column of Sephadex LH-20. The 4 radioactive fractions thus obtained by methanol gradient were about 44% of the total radioactivity. By thin-layer chromatography, 1 ninhydrin-positive and fluorescent spot was detected in each of the 2 radioactive fractions. The gas chromatographic and mass spectrometric analyses suggest that 1 of these 2 fractions contains a BP-diol epoxide-lysine conjugate.