CD43 is a hematopoietic cell antigen whose distribution includes T lymphocytes, plasma cells, neutrophils, and platelets. Although it has been detected on peripheral blood monocytes, its expression by other mononuclear phagocytes has not been well documented. Possible changes in monocyte/macrophage CD43 expression in response to inflammation are also poorly defined. To examine these questions, the expression of CD43 by rat peripheral blood monocytes and both resident and elicited peritoneal macrophages was examined. By flow cytometry with two anti-CD43 monoclonal antibodies, blood monocytes were found to express large amounts of surface CD43, whereas surface CD43 expression by resident peritoneal macrophages was negligible. Peritoneal macrophage populations elicited by intraperitoneal injection of thioglycollate were uniformly positive for surface CD43, although the level of expression was lower than that found on monocytes. By labeling resident macrophages with a fluorescent tracer dye, this phenotypic shift was found to reflect an influx of CD43-positive elicited macrophages coupled with a disappearance of CD43-negative resident cells. Evidence from both flow cytometry and Western blotting studies suggests that the CD43 expressed by elicited peritoneal macrophages is less heavily sialylated than that expressed by blood monocytes. These findings, coupled with recent evidence that CD43 influences cellular adhesion, indicate that differential expression of CD43 may play a role in monocyte/macrophage trafficking.