The rate and level of DnaK-dependent refolding of heat-inactivated Vibrio fischeri luciferase in the clp A mutant (clp A:: kan) were considerably lower then in wild-type cells. The decline in refolding level progressed with increasing heat inactivation time. A mutation of clp P had no influence on the kinetics and level of luciferase refolding. Approximately equal amounts of the DnaKJE chaperone were synthesized upon heat shock induction in E. coli clp A + and E. coli clpA::kan cells. It was assumed that, like homologous chaperone ClpB, ClpA is involved in disaggregation of denatured proteins, increasing the refolding efficiency. This in vivo phenomenon occurred only upon a prolonged incubation of cells at a higher temperature, which led to the formation of large protein aggregates that were poorly refoldable by the DnaKJE system.