The mechanisms which generate heterogeneity amongst resident tissue macrophages (M phi) are poorly understood. In a previous study we described a novel mouse M phi haemagglutinin, which binds unopsonized sheep erythrocytes. This sheep erythrocyte receptor (SER) is expressed at high levels on stromal M phi from bone marrow and lymph nodes, but at low levels on M phi from serous cavities and broncho-alveolar spaces. In this paper we demonstrate that a species-restricted factor in mouse serum is required in vitro for optimal maintenance of SER on resident bone marrow M phi and for its induction on M phi populations which normally lack this receptor. Using thioglycollate-elicited peritoneal M phi, induction of SER by mouse serum was dose-dependent, reached maximal levels by 3-4 days, required the continuous presence of mouse serum, and was fully reversible. Re-expression following trypsinization was inhibited by cycloheximide, showing that protein synthesis by M phi was necessary. Using a quantitative microtitre plate assay to measure levels of the inducing activity (SER-IA) in different samples, it was found to be heat-labile, non-dialysable, precipitable by polyethylene glycol and inactivated at pH 4 but not at pH 9.6. On gel filtration of mouse serum, a single major peak of activity was obtained with an apparent MW of around 70,000. SER-IA appears to be unrelated to a variety of factors and cytokines which affect M phi function, including colony-stimulating factor-1 (CSF-1). The possible role of SER-IA in regulating the differential expression of SER in vivo is discussed.