The folding of in vitro synthesized outer membrane protein PhoE of Escherichia coli was studied in immunoprecipitation experiments with monoclonal antibodies which recognize cell surface-exposed conformational epitopes. The signal sequence appears to interfere with the formation of these conformational epitopes, since a mutant PhoE protein which lacks the majority of the signal peptide could be precipitated four times better than the wild type precursor. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the immunoprecipitated PhoE protein revealed that part of the immunoprecipitated PhoE was present as a heat-modifiable form of the protein which migrated faster in the gels than the completely denatured protein. This form of the protein probably represents a folded monomer which might be an intermediate in the assembly of the protein. Outer membrane vesicles were required to induce the formation of small amounts of heat-stable trimers, the functional form of the protein in vivo.