To determine whether expression of type 1 pili varies during the course of Escherichia coli infection in vivo, mice were injected intraperitoneally with 5 x 107 CFU of piliated or nonpiliated phase variants per ml, and the degree of piliation was measured in peritoneal exudate by an enzyme-linked immunosorbent assay inhibition method. In the animals challenged with the piliated bacteria, the numbers of organisms increased a log over 9 h and the amount of pilus antigen decreased from 3 to 0.075 μg/10 bacteria. After a 4-h delay, nonpiliated bacteria also increased by one log over 9 h; however, the amount of piliation remained virtually undetectable. Piliated E. coli were more virulent than nonpiliated variants in this model (50% lethal dose of 7.5 x 106 versus 3 x 107, respectively). The difference was significantly reduced by prior passive immunization with rabbit serum containing high titers of antipili antibody. Piliated bacteria adhered in significantly greater numbers to isolated mouse peritoneal membranes than did nonpiliated variants (15,400 ± 2,700 versus 1,300 ± 700 bacteria/mm2, respectively; P = 0.05). Adherence was inhibited by the presence of 0.1 M alpha methyl mannose (1,500 ± 1,800 bacteria/mm2, P = 0.01). These results confirm the results of previous qualitative studies showing that phase variation of type 1 pili occurs in vivo and suggest that these pili may confer an initial advantage for growth of E. coli in the peritoneal cavity, presumably by fostering colonization of the peritoneal serosal surface.