Cell-mediated cytotoxicity (CTX) of meningioma patients towards meningioma cells and fibroblasts was studied in vitro before and after surgery, using the 3H-proline microcytotoxicity test. When incubated with allogeneic target cells before surgery, lymphocytes from three out of seven donors showed a specific destruction of meningioma tissue, and two were cytotoxic on both types of targets. Five patients were tested against their own tumour and against skin fibroblasts; this was feasible by keeping their lymphocytes frozen in liquid nitrogen until their target cells grew as suitable monolayers in vitro. Three patients showed a specific cytotoxic response. After excision of the tumour a gradual loss of this reactivity was observed in both allogeneic and autologous systems. Sera of three patients, whose lymphocytes were not reactive against their own tumour, induced antibody-dependent cellular cytotoxicity (ADCC) towards autologous tumours if normal effector cells were used. This type of response was detectable mainly in postoperative sera, and could not be elicited in autologous lymphocytes. On the contrary, autologous sera inhibited CTX of meningioma effector cells. The data suggest that meningiomas can induce a complex immunological response in the host, which is dependent on the presence or absence of a large tumour burden.