The recognition mechanism of xenogenic red cells by mouse macrophages was studied by using established cell lines. Approximately 30% of cell line cells Mm1 which lack la antigen, as well as of thioglycollate-induced peritoneal macrophages from SL/Am mice (TGC-M phi) could ingest unopsonized quail red cells (QRC). In contrast, an undifferentiated type of cell line, M1-, and another type of macrophage cell line, Mk1-C, possessing accessory cell activity in association with the expression of la antigen, had no phagocytic activity for QRC. Approximately 80% of Mm1 cells, as well as TGC-M phi formed rosettes with QRC, whereas M1- and Mk1-C cells did not; indicating that specific binding sites for QRC are expressed on a large portion of Mm1 and TGC-M phi but not on M1- and Mk1-C cells. No requirement of divalent cation (Mg++, Ca++) and metabolic energy was observed for rosette formation between Mm1 cells and QRC. Protease treatment of Mm1 cells eliminated the rosetting activity, whereas periodate oxidation of glycosidase treatment slightly enhanced this activity, suggesting the involvement of surface protein in binding sites of Mm1 cells. In contrast to these findings on Mm1 cells, binding components of QRC were sensitive to periodate oxidation or neuraminidase treatment but resistant to protease, suggesting that the terminal sialic acid residues of carbohydrate of QRC are recognized by Mm1 cells. Furthermore, N-acetylneuraminic acid (NeuNAc) inhibited the rosette formation and promoted the dissociation of rosettes already formed. N-Acetylneuramin lactose (Neu-NAc-Lact) was more efficient in rosette inhibition than NeuNAc. These sugars also blocked the phagocytosis of QRC by Mm1 cells but had no effect on either Fc-mediated phagocytosis or latex ingestion. These results suggest that phagocytosis of QRC by murine macrophages is mediated by protease-sensitive binding sites recognizing terminal sialic acid residues of QRC in conjunction with additional carbohydrates.