Escherichia coli DegP is an inducible serine protease which is involved in the breakdown of abberant proteins arising in the periplasmic compartment. Overexpression of alkaline phosphatase (PhoA) increased transcription of degP by twofold. To examine the significance of its induction, we overexpressed PhoA in a mutant strain deficient in the degP gene. Upon PhoA overexpression, the degP mutant produced a smaller amount of active PhoA, about one half of the enzymatic activity of its isogenic wild-type strain, and accumulated a larger amount of its precursor, indicating that degP is required for efficient export of overexpressed PhoA. Pulse-chase experiment showed that PhoA overexpression in the absence of degP causes a severe defect in the export of several proteins tested. Examination of the synthesis and the accumulation of the phoA gene products revealed that a part of them, synthesized in the wild-type strain, undergoes relatively rapid proteolysis and that degP is necessary for such a process. From these results, we discuss a possible role of DegP in facilitating protein export under stress conditions.