Dendritic cells (DCs) might be partly responsible for the defective immune response in tumor bearing hosts, but no study in osteosarcoma patients is still available. Therefore, we investigated in vitro whether human osteosarcoma cell lines have an inhibitor effect on different types of DCs: CD14+DCs, DC1 and DC2. DCs derived from healthy donors were cultured with osteosarcoma cell lines and appropriate cytokine cocktails and analysed for the expression of co-stimulatory molecules (CD40, CD80, CD83, CD86, HLA-DR). Each interaction resulted in a lower phenotypic expression of the DCs maturation markers, especially on DC2. Moreover, the addition of various cytokines and compounds (rhIL-12, CD40L, Indometacin) induced the DC1 and DC2 subsets towards the Th1 pattern as shown by ELISA. Osteosarcoma highly interferes with an in vitro DCs immune function as antigen presenting cells. The understanding of tumor biology underlines the need for a specific osteosarcoma immunotherapy able to reverse this immune-surveillance inhibition.