Mycoplasma contamination of cultured cells may cause considerable interference with their behaviour and metabolism. Whether or not mycoplasma are actually taken up by cells in vitro has long been a matter of dispute. In the present study mycoplasma were shown to be phagocytosed by feline lung fibroblasts and to end up in the lysosomal vacuome of the cells, where the organisms were only partially degraded by the hydrolytic enzymes leading to a rapid (within a few days) accumulation of secondary lysosomes of the residual body variety. It is thus obvious that studies on the structure and function of the lysosomal vacuome in cultured cells are possible only if precautions are taken to avoid that the cultures become contaminated by mycoplasma.