In this paper we describe immunocytochemical detection of PhoE pore protein in the outer membrane of E. coli K-12 cells in dependence of a variety of labelling approaches. Immuno-gold labelling on ultrathin cryosections showed a uniform, dense labelling of the outer membrane of all cells. However, using immunofluorescence, whole-mount or freeze-etch labelling methods, labelling was limited to less than 5% of the cell population. In order to understand this phenomenon, immunoincubated cells exhibiting less than 5% labelling were processed for cryo-ultramicrotomy. Reincubation of the sections with antibody and probe resulted in labelling of all of the cells. If an E. coli Gal-U mutant strain, defective in the lipopolysaccharide (LPS) carbohydrate chain length, was used, each approach rendered 100% labelling. From these results it is concluded that the antigenic sites of the PhoE pore protein are not accessible in intact 'wild-type' cells due to steric hindrance caused by the LPS carbohydrate chains. In cryosections this steric hindrance is partly precluded since antigenic determinants are exposed at the section surface in transversely cut membranes. Our results emphasize the necessity to compare results obtained by means of several, basically different approaches.