Phenotypic heterogeneity between macrophages in situ has been reported, using different macrophage-specific monoclonal antibodies. Microenvironmental factors, which may play an important role in inducing this heterogeneity were studied in a local inflammatory response in vivo. Our results show that peritoneal exudate macrophages, formed during Thioglycollate induced chronic inflammation, acquire expression of the dendritic cell markers NLDC-145 and 6D2. In contrast to these phenotypic characteristics, NLDC-145+/6D2+ exudate cells do not function like dendritic cells in an allogeneic-MLR, but are suppressive when compared with steady state peritoneal cells. The expression of these membrane markers on exudate cells is discussed.