Mutations at several different chromosomal locations affect expression of the major outer membrane porin proteins (OmpF and OmpC) of Escherichia coli K12. Those that map at 21 and 47 minutes define the structural genes for OmpF and OmpC, respectively. A third locus, ompB, is defined by mutations that map at 74 minutes. The ompB locus contains two genes whose products regulate the relative amounts of ompF and ompC expression. One of these genes, ompR, encodes a positive regulatory protein that interacts at the ompF and ompC promoters. Mutations in ompR exhibit an OmpF- OmpC- or an OmpF+ OmpC- phenotype. The product of the second gene, envZ, affects regulation of the porin proteins in an unknown manner. Previously isolated mutations in envZ exhibit an OmpF- OmpC+ phenotype and also have pleiotropic effects on other exported proteins. In the presence of local anaesthetics such as procaine, wild-type strains exhibit properties similar to these envZ mutants, i.e. OmpF- OmpC+. Using ompF-lac fusion strains, we have exploited this procaine effect to isolate two new classes of envZ mutations. One of these classes exhibits an OmpF+ OmpC- phenotype. The other allows expression of both OmpF and OmpC but alters the relative amounts found under various growth conditions. Like previously isolated envZ mutations, these also affect regulation of other exported proteins, such as lambda receptor. These results permit a more detailed analysis of the omp regulon and they may shed light on one of the mechanisms by which local anaesthetics exert their effect.