Pellets obtained by the centrifugation of unfiltered or filtered fetal bovine serum (FBS) contained, among other things, numerous elongate and spherical microvesicles, ranging from 30 to 60 nm. They possessed a trilaminar (unit membrane) envelope and a moderately electron-dense core. Vesicles of multivesicular bodies and microvesicles associated with secretory epithelial cells and human lymphoblastoid cells in suspension culture possessed the same structure and were in the same size range as the serum microvesicles. These structures were either normal cell components or they represented the breakdown products of normal cell components and should not be confused with viruses. Though microvesicles of FBS may have been involved in the tissue culture material, at least some of the microvesicles associated with, and phagocytosed by, human lymphoblastoid cells in suspension and those in the lumen of acini of the baboon prostate were derived from the breakdown of the cristae of mitochondria in degenerating cells.