The interaction of aflatoxin B1 with DNA was investigated. In the presence of native DNA the absorption spectrum of the toxin showed an obvious spectral shift in the region of 300-420 nm with an isosbestic point at 376 nm. DNA-bound aflatoxin is hydrolyzed in alkaline media more easily than the free toxin. The hydrolized form has a labile structure and can decompose further. The bound toxins are easily dissociated by heat as well as by salt, and all toxin molecules are released from DNA which remains double-stranded. Aflatoxin can bind to thermally denatured DNA as well, with an accompanying spectral shift which depends on the particular preparation of denatured DNA, and there was an isosbestic point at 380 nm. The complex of toxin and denatured DNA was stabilized by salt up to 0.1 M. Thus it was concluded that aflatoxin was bound with denatured DNA in a different form from native DNA. The number of binding sites of DNA was estimated by constructing Scatchard plots based on both spectral analysis and equilibrium dialysis. However, these show no definite value but fall in the region of 0.07 to 0.013, that is one toxin molecule per 80-140 nucleotide of native DNA. Several lines of evidence suggest the possibility that aflatoxin exists in aqueous solution as aggregates. The mechanism of the binding was discussed. It is noteworthy that the number of binding sites of DNA doubles by the presence of histones.