Expression of B and T lymphocyte receptors has been studied in seven cases of reticulum cell sarcoma. In one case, surface receptors and tests of phagocytic function demonstrated the histiocytic origin of the neoplastic cells. In four cases, tumour cells expressed both B and T lymphocyte markers (two cases) or showed a normal pattern of expression of B and T lymphocyte markers. In the other two cases, lymphocyte receptors were not detected, and there was no evidence of phagocytic function: this class of receptor-silent tumours is of uncertain pathogenesis. The significance of these observations is discussed.