Ficolins are a group of multimeric proteins that contain collagen-like and fibrinogen-like (FBG) sequences. Three types of ficolins have been characterized: H-, L- and M-ficolins. Both H- and L-ficolins have demonstrated lectin activities. In the present study, the FBG domain of M-ficolin was expressed and shown to bind to N-acetyl-D-glucosamine. M-ficolin mRNA was expressed in monocytes but not in the more differentiated macrophages and dendritic cells. By flow cytometry, surface biotinylation and immunoprecipitation, we showed that M-ficolin was associated with the surface of promonocytic U937 cells. M-ficolin transiently expressed in COS-7 cells was also clearly detected on the cell surface by immunoprecipitation. By flow cytometry, M-ficolin was detected on peripheral blood monocytes but not on lymphocytes or granulocytes. Immobilized rabbit anti-M-ficolin F(ab')2 mediated U937 cell adhesion, and the antibody also inhibited phagocytosis of Escherichia coli K-12 by U937 cells. Therefore, M-ficolin might act as a phagocytic receptor or adaptor on circulating monocytes for micro-organism recognition and may potentially mediate monocyte adhesion.