AbstractImmunization of C57BL/6 mice with tumor-derived membrane-proteins encapsulated in sized liposomes (0.2μg/mouse) and composed by phosphatidylcholine or sphingomyelin, significantly reduced the mean values of spontaneous lung metastasis from both B16 (0.7±0.5 and 1.2±0.6, respectively) and 3LL (4.8±2.5 and 7.2±4.1, respectively) tumors, with respect to control (HEPES) groups (4.8±1.1 and 19.0±4.4, respectively). However, no significant antimetastatic effect was observed using free tumor-derived proteins (2μg/mouse) or liposome vehicle alone. Specific humoral immune response after the vaccination was studied by flow cytometry of tumor cells incubated with a pooled sample from each group of immunized mice and FITC-conjugate antimouse immunoglobulins. The results showed that the highest number of positive tumor cells was identified using sera from immunized mice with sized liposomes encapsulating tumor-derived proteins whereas the immunization with the protein fraction in free form failed to induce this effect. In addition, an increased cytotoxicity towards 3LL and B16 tumor cells can also be observed when tumor cells were incubated with spleen effector cells plus specific immunosera. In conclusion, our results show that antitumor active vaccination, using sized liposomes as adjuvants, induces an antitumor host response and a significant inhibition of tumor progression.