Common enterobacterial antigen (CA) and type-specific E. coli O-antigen were studied in rat peritoneal macrophages by means of immunofluorescence technique. At different time intervals after intraperitoneal injection of killed or viable E. coli O4, macrophages were harvested from the peritoneal fluid and the number of macrophages in which CA or E. coli O4-antigen, respectively, could be demonstrated was recorded. During the first few days, equal numbers of peritoneal macrophages were found to contain CA and E. coli O4-antigen, but on days 7—14 and onwards during the remaining experiment, i.e. throughout 8 weeks, CA was detected in fewer macrophages than E. coli O4-antigen, indicating a lesser degree of persistence of CA than of E. coli O4-antigen in the macrophages after phagocytosis of the bacteria. The results correspond with those previously obtained in experimental pyelonephritis in rats.