The gene locus known as mtr confers resistance to hydrophobic dyes, detergents, and antibiotics. It has been suggested previously that the host environment is important in the selection of gonococcal strains with this outer membrane phenotype, and thus that strains with mtr gene loci should predominate in environments high in hydrophobic molecules. Furthermore, resistance to hydrophobic molecules has been related to a sevenfold increase in a minor outer membrane protein. To test these suggestions the outer membrane phenotypes of 61 strains of Neisseria gonorrhoeae were identified using 27 rectal isolates from homosexual men and 34 urethral isolates from heterosexual men who were matched for age. The cell envelope phenotype of each strain was identified on the basis of resistance to various hydrophobic compounds. The results were compared with the protein profiles of these strains on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE); no significant correlation was found.