Mutant Escherichia coli strains in which export of the LamB protein (coded for by the lamB gene) to the outer membrane of the cell is prevented have been described previously. One of these mutant strains contains a small (12-base pair) deletion mutation within the region of the lamB gene that codes for the NH2-terminal signal sequence. In this mutant strain, export but not synthesis of the LamB protein is blocked. We have isolated pseudorevertants that restore export of functional LamB protein to the outer membrane. DNA sequence analysis showed that two of the revertants contain a point mutation in addition to the original deletion. These point mutations lead to amino acid substitutions within the signal sequence. Our results indicate that these secondary mutations efficiently suppress the export defect caused by the deletion mutation. Analysis of the secondary structure of the wild-type, mutant, and pseudorevertant LamB signal sequences suggests that the secondary mutations restore export by allowing the formation of a stable alpha-helical conformation in the central, hydrophobic region of the signal sequence.