Properties of purified F1-ATPase from Escherichia coli mutant strain AN484 (uncD412) have been studied in an attempt to understand why the amino acid substitution in the beta-subunit of this enzyme causes a tenfold reduction from normal MgATP hydrolysis rate. In most properties that were studied, uncD412 F1-ATPase resembled normal E. coli F1-ATPase. Both enzymes were found to contain a total of six adenine-nucleotide-binding sites, of which three were found to be non-exchangeable and three were exchangeable (catalytic) sites. Binding of the non-hydrolysable substrate analogue adenosine 5'-[beta gamma-imido]triphosphate (p[NH]ppA) to the three exchangeable sites showed apparent negative co-operativity. The binding affinities for p[NH]ppA, and also ADP, at the exchangeable sites were similar in the two enzymes. Both enzymes were inhibited by efrapeptin, aurovertin and p[NH]ppA, and were inactivated by dicyclohexylcarbodi-imide, 4-chloro-7-nitrobenzofurazan and p-fluorosulphonyl-benzoyl-5'-adenosine. Km values for CaATP and MgATP were similar in the two enzymes. uncD412 F1-ATPase was abnormally unstable at high pH, and dissociated into subunits readily with consequent loss of activity. The reason for the impairment of catalysis in uncD412 F1-ATPase cannot be stated with certainty from these studies. However we discuss the possibility that the mutation interrupts subunit interaction, thereby causing a partial impairment in the site-site co-operativity which is required for 'promotion' of catalysis in this enzyme.