Background: Extracellular exosomes contain various functional elements. Results: Exosomal Tax protein causes phenotypic changes in uninfected cells. Conclusion: Exosomes may play critical roles in extracellular delivery of oncogenic material derived from HTLV-1-infected cells. Significance: Exosomal delivery of Tax and other putative oncogenic components produced during HTLV-1 infection potentially contributes to pathogenesis of adult T-cell leukemia, myelopathy, or tropical spastic paraparesis.