A universal influenza virus vaccine that does not require frequent updates and/or annual immunizations will offer significant advantages over current seasonal flu vaccines. The highly conserved influenza virus A M2 membrane protein has been previously suggested as a potential antigen target for such a vaccine. Here, we report systematic evaluation of M2 peptide conjugate vaccines (synthetic peptides of M2 extracellular domain conjugated to keyhole limpet hemocyanin (KLH) or Neisseria meningitidis outer membrane protein complex (OMPC)) in mice, ferrets, and rhesus monkeys. The conjugate vaccines were highly immunogenic in all species tested and were able to confer both protection against lethal challenge of either H1N1 or H3N1 virus in mice and reduce viral shedding in the lower respiratory tracts of mice and ferrets. The protection against lethal challenge in mice could also be achieved by passive transfer of monkey sera containing high M2 antibody titers. In addition, we showed that M2 antisera were cross reactive with M2 peptides derived from a wide range of human influenza A strains, but they failed to react with M2 peptides of the pathogenic H5N1 virus (A/Hong Kong/97). The data presented here will permit better understanding of the potential of an M2-based vaccine approach.