The concentration of mast cells was estimated, and their distribution noted, in 132 benign nerve sheath tumours. With few exceptions, neurofibromas were characterised by a high concentration of diffusely distributed mast cells. In neurilemmomas fewer mast cells were present and these were restricted to Antoni type-B tissue. Based on these observations, and on the behaviour of mast cells in injured nerves, it is postulated that neurilemmomas are true neoplasms of Schwann cells sometimes incorporating reactive endoneurial tissue (Antoni type-B tissue). Neurofibromas probably consist of reactive endoneurial tissue and may not be true neoplasms. The distinction between neurofibromas and neurilemmomas thus seems to be a real one based on a difference in histogenesis.