FLK cell culture chronically producing bovine leukemia virus (BLV) and spontaneously contaminated with mycoplasma (M. sp.) was studied by electron microscopy. Information on the features of the ultrastructural organization of M. sp. and its direct relationships with BLV was obtained. BLV virions are capable of adsorbing on mycoplasma surface and penetrate inside, apparently in the course of phagocytosis. In the cytosole of M. sp. free-lying BLV virions devoid of the limiting membrane were found. In FLK cells both agents were regularly produced in high amounts causing no signs of death of the cell culture itself. From the foregoing, the FLK cells--BLV-M. sp. system appears to be quite balanced.