Extracellular type I tumor necrosis factor receptors (TNFR1) are generated by two mechanisms, proteolytic cleavage of TNFR1 ecto-domains and release of full-length TNFR1 in the membranes of exosome-like vesicles. Here, we assessed whether TNFR1 exosome-like vesicles circulate in human blood. Immunoelectron microscopy of human serum demonstrated TNFR1 exosome-like vesicles, with a diameter of 27 - 36 nm, while Western blots of human plasma showed a 48-kDa TNFR1, consistent with a membrane-associated receptor. Gel filtration chromatography revealed that the 48-kDa TNFR1 in human plasma co-segregated with LDL particles by size, but segregated independently by density, demonstrating that they are distinct from LDL particles. Furthermore, the 48-kDa exosome-associated TNFR1 in human plasma contained a reduced content of N-linked carbohydrates as compared to the 55-kDa membrane-associated TNFR1 from human vascular endothelial cells. Thus, a distinct population of TNFR1 exosome-like vesicles circulate in human plasma and may modulate TNF-mediated inflammation. Published by Elsevier Inc.