M2 is an integral protein of influenza A virus that functions as an ion channel. The ratio of M2 to HA in influenza A virions differs from that found on the cell surface, suggesting selective incorporation of M2 and HA into influenza virions. To examine the sequences that are important for M2 incorporation into virions, we used an incorporation assay that involves expressing M2 from a plasmid, transfecting the plasmid into recipient cells, and then infecting those cells with influenza virus. To test the importance of the different regions of the protein (extracellular, transmembrane, and cytoplasmic) in determining M2 incorporation, we created chimeric mutants of M2 and Sendai virus F proteins, exchanging corresponding extracellular, transmembrane, and cytoplasmic domains. Of the six possible chimeric mutants, only three were expressed on the cell surface. Of these three chimeric proteins, only one mutant (with the extracellular domain from M2 and the rest from F) was incorporated into influenza virions. These results suggest that the extracellular domain of M2 is important for its incorporation into virions.