The probable ultimate urinary bladder carcinogen, N-hydroxy-2-naphthylamine (N-OH-2-NA), reacted with nucleic acids and proteins under mildly acidic conditions (pH 5) to form covalently bound derivatives. The extent of reaction was in the order: Polyguanylic acid greater than DNA approximately protein greater than rRNA greater than tRNA greater than polyadenylic acid approximately polyuridylic acid greater than polycytidylic acid. At pH 7, appreciable reaction occurred only with protein. Enzymatic hydrolyses of the DNA, which contained 1.5 naphthyl residues/1,000 nucleotides, yielded 3 nucleoside-arylamine adducts. From chemical, UV, nuclear magnetic resonance, and mass spectrometric analyses, the adducts were identified as 1-(deoxyguanosin-N2-yl)-2-NA, 1-(deoxyadenosin-N6-yl)-2-NA, and a purine ring-opened derivative of N-(deoxyguanosin-8-yl)-2-NA, tentatively identified as 1-[5-(2-6-diamino-4-oxopyrimidinyl-N6-deoxyriboside)]-2-(2-naphthyl)urea. Preliminary experiments with a dog given [3H]2-NA suggested the presence of these adducts in vivo. The properties of adducts derived from N-OH-1-NA and N-OH-2-NA and their possible roles in the initiation of carcinogenesis are discussed.