RESUMEN
1. Outer-membrane protein patterns of Escherichia coli recovered from the peritoneal cavities of infected guinea pigs and grown in medium M9 containing 2,2'-dipyridyl were studied by polyacrylamide gel electrophoresis (SDS-PAGE) to determine whether in vivo conditions of growth affected the expression of these bacterial surface proteins. 2. Eleven strains of septicemic E. coli studied in vitro under conditions of iron restriction expressed iron-regulated outer-membrane proteins, mainly the protein of approximately 74 kDa, whereas avirulent strains grown under similar conditions did not present the 74-kDa protein. 3. These results show the distribution of iron-regulated outer-membrane proteins among avian E. coli and suggest that the protein of approximately 74 kDa may be important for the virulence of these strains.