Oligomannose-coated liposomes (OMLs), containing entrapped antigens, serve as effective antigen delivery vehicles and as a novel adjuvant to induce antigen-specific cellular immune responses. However, in vitro activation of antigen-presenting cells (APCs) by OMLs has not yet been demonstrated. In this paper, we found that OMLs can deliver the antigens and the stimulatory signals into inflammatory monocytes in vitro, leading to differentiation of the cells to mature APCs. When OMLs were co-cultured with peripheral blood mononuclear cells from C57BL/6 mice in the presence of mouse serum, OMLs were preferentially incorporated into both Ly6Chigh monocytes and Ly6Clow monocytes, which are referred to as murine inflammatory and resident monocytes, respectively. The expression of CD11c, CD80, CD86, CCR7, and MHC class II on the Ly6Chigh monocytes was significantly enhanced during the 24 h after OML uptake, whereas upregulation of these molecules on the Ly6Clow monocytes was limited. In addition, the antigenic peptide of OVA encased in OMLs was presented on MHC class I of only Ly6Chigh monocytes. Furthermore, OVA-encasing OML-ingesting monocytes can activate CD8+ T cells from OT-1 mice, suggesting that antigens encapsulated in OMLs were cross-presented in inflammatory monocytes. Adoptive transfer of the monocytes that engulf OVA-encasing OMLs led to induction of an antigen-specific Th1 immune response in mice. Taken together, mature APCs can be generated from inflammatory monocytes in peripheral blood by ex vivo treatment of the cells with OMLs without any additional stimuli.