Cell-mediated cytotoxicity (CTX) of meningioma patients was assessed postoperatively by a [3H]proline microcytotoxicity test. Autologous and allogeneic tumour cells were used for prelabelling with isotope and peripheral blood lymphocytes added in a ratio of 200:1. After 60 hg the plates were washed and residual CMP counted. Control target cells consisted of normal skin fibroblasts. CTX was calculated in percentage reduction compared to cultures incubated with control lymphocytes. Specific CTX on meningioma cells (i.e. not destroying control cells) greater than 20% was considered 'positive' if significant at P less than 0-05. Fifteen of twenty-three meningiomas showed specific CTX (65%). Among eight CNS tumours of different type and thirteen non-malignant diseases and normals only three (14%) were specifically cytotoxic for meningioma cells. A cross-reaction could be demonstrated between autologous and allogeneic meningioma target cells. However, no activity of lymphocytes from patients with meningiomas on glioblastoma cells and foetal brain tissue could be found at the ratio used for evaluation. Evidence is presented indicating that a cellular immune response as measured in the microcytotoxic test may be dependent on a residual or recurrent tumour in the body.