Influenza viruses produce pleomorphic virions, which has made it difficult to study their composition. Hutchinson et al. used a mass spectrometry approach to determine and quantify the protein composition of influenza virions. They show that the core architecture of virions is conserved despite variation in proteins that are encoded by different hosts and influenza viruses. In addition, the authors found that the conserved core structure is elaborated with host-specific proteins from avian or mammalian hosts, which suggests that distinct virions are associated with each host and that virion composition may need to switch for the virus to cross species barriers. The protein composition of virions exhibited similarities to that of purified host exosomes, which suggests that the virus might hijack the host pathways used for vesicle formation to generate new virions.