The effect of adenine ribonucleotides on the stability of Escherichia coli dnaB protein in cellular crude extracts was studied. Stabilization of dnaB protein by ATP or ADP, but not by AMP, was manifested in that (i) the activity and yield of wild type dnaB protein is enhanced in the presence of ATP, (ii) the dnaB protein of E. coli dnaB mutants, such as groPB and dnaB252/ColE1::dnaC+, which is inactive in a dnaB complementation assay, can be isolated in active form in the presence of ATP or aDP, (iii) ATP or ADP protect the dnaB protein of an E. coli dnaBts mutant from inactivation at 37 degrees C, and (iv) inactive groPB and dnaBts protein can be reactivated partially by ATP. Thus, the stabilizing effect of ATP and ADP can be exploited for the isolated of otherwise inactive or labile mutant dnaB proteins.