Outer membrane protein patterns (Omp) of Escherichia coli obtained directly from the urine of bacteriuric patients without passage on artificial culture media (ACM) were studied by polyacrylamide gel electrophoresis (SDS-PAGE) in an effort to determine whether in vivo conditions of growth affected the expression of these bacterial surface structures. Seventeen strains studied showed two distinct Omp patterns: one protein band appeared at the level of porin proteins (40 kDa) in both patterns, but Omp A protein was at the level of 36 kDa in the first pattern and a new protein was observed at 21.5 kDa in the second pattern suggesting that it is a fragment of Omp A. High molecular weight proteins were also observed in most of the strains and this finding was related to lack of free iron when the same strains were grown under iron restricted conditions in vitro. The same strains grown in pooled urine from normal females showed the first pattern mentioned above. Comparative growth on ACM of urinary strains and E. coli strains isolated from blood, feces and wounds showed an increase in the number of porins expressed (from 1 to 2 or 3, with some variability observed between strains). Differences in osmolality between pooled urine and ACM used, plus in vitro studies varying the osmolality of culture media, showed that osmolality accounted for differences in the number of porins expressed: porin expression decreased in urine the ACM of high osmolality, suggesting that the same phenomena occurred in vivo. It is concluded that host factors including low availability of iron and high osmolality present in the urinary tract influence the expression of several E. coli surface proteins. These proteins may relate to the ability of E. coli to colonize and invade the urinary tract by regulating the physiologic and/or metabolic state of the bacterial cell favoring survival of the organism in a hostile environment. Specific immune responses directed against porins could influence the outcome of this host-parasite interaction.