Long particles, 75 nm in diameter, some branching and some curled at their ends, were found within vacuoles in "L" cells. These could be easily transmitted to Vero cells, which like the "L" cells were killed. The particles were up to 20 microns in length with hollow cores 37 nm in diameter surrounded by a smooth unit membrane from which no spikes protruded. The particles could not be assigned to any known taxon.