To investigate whether bile within the abdominal cavity changes the morphologic expression of peritonitis, rats received intraperitoneal injections of saline, bile, Escherichia coli (3 x 10(8) colony-forming units), or E. coli and bile (n = 10 in each group). Specimens were taken from the parietal and visceral peritoneum for light microscopy and scanning electron microscopy, 10 h after induction of peritonitis. Bile alone increased the number of inflammatory cells, especially of the monocytic type. After E. coli alone the monocytic cell response was less pronounced, whereas the polymorphonuclear leukocyte response was comparatively large in the visceral peritoneum. Addition of bile in E. coli peritonitis caused a substantial increase in polymorphonuclear leukocytes, especially in the visceral peritoneum, and in the depth of the inflammatory cell reaction. At scanning electron microscopy mesothelial cells appeared normal after bile alone, had reduced numbers of microvilli after E. coli alone, and were denuded or separated, without microvilli, after E. coli and bile. It is concluded that the inflammatory effects of E. coli are greatly enhanced in the presence of bile.