When 8-bromoadenosine was incubated with cysteine at pH 7.2 and 37°C, an exclusive product was generated. This product was identified as a cysteine substitution derivative of adenosine at the 8 position, 8- S -L-cysteinyladenosine. The reaction accelerated as pH increased from mildly acidic to basic conditions. The isolated cysteine adduct of adenosine decreased with a half-life of 15 h at pH 7.2 and 37°C. Similar results were obtained for the incubation of 8-bromo-2′-deoxyadenosine and 8-bromoadenosine 3′,5′-cyclic monophosphate with cysteine. These results suggest that 8-bromoadenine in nucleotides, RNA, and DNA can react with thiols, resulting in adducts under physiological conditions.