The hflA locus of Escherichia coli specifies a multisubunit protease that selectively degrades the cII transcriptional activator of phage lambda. The regulated turnover of cII is critical for the choice between the lytic and lysogenic pathways of viral development. Previous cell fractionation work has indicated that HflA is associated with the inner membrane fraction. We have sought to demonstrate that the HflA protease is localized in the cell membrane of intact cells. To achieve this goal, we have combined electron microscopy of thin-sectioned E. coli cells with antibody tagging by a colloidal gold label. Using antibody to purified HflA protein, we have found preferential membrane labeling for hflA+ cells but not for hflA mutant cells. We conclude that HflA protease is localized in the cell membrane. The membrane location for HflA protein may serve as a component of a targeting mechanism to limit the action of the regulatory protease to selected cytoplasmic proteins.