The reaction of the E. coli chaperonin GroEL (cpn 60) with the ATP analogue 2',3' oxidised ATP (oATP) has been studied. Treatment with the reagent leads to loss of the ATPase activity of GroEL in a pseudo-first-order fashion; this can be prevented by inclusion of ATP in the reaction mixture. Measurements of the stoichiometry of the reaction indicate that the loss of activity corresponds to the incorporation of about one oATP per subunit of GroEL. From analysis of the sequences of modified peptides it is proposed that the reaction probably occurs with one or both of the two cysteines Cys-457 and Cys-518, although the instability of the adduct(s) makes a definite identification of the site(s) of reaction difficult. The involvement of Cys side chains in the reaction with oATP was confirmed by using Nbs2 (5,5'-dithiobis(2-nitrobenzoate)) to estimate thiol groups in both modified and unmodified GroEL.