Carcinoembryonic antigen (CEA) in plasma, cerebrospinal fluid (CSF) and cystic fluid of patients with various intracranial tumors were determined by a radioimmunoassay using Dainabot and Roche CEA Kits. Slightly elevated levels of plasma CEA were noted in 6 (4%) out of 161 (6/161) patients with primary brain tumors: gliomas, 3/67; pituitary adenomas, 0/30; meningiomas, 0/28; acoustic neurinomas, 1/12; pineal tumors, 2/12 (germinomas, 1/8; teratoma, 0/1 ; teratoblastoma, 0/1 ; embryonal carcinoma, 1/1 ; not verified histologically, 0/1); craniopharyngiomas, 0/6; hemangioblastomas, 0/2; epidermoid tumor, 0/1; reticulum cell sarcoma, 0/1; choroid plexus papilloma, 0/1; chondroma, 0/1. The plasma CEA levels of these 6 patients varied from 2.6 to 3.8 ng/ml. On the other hand, 17 (37%) out of 46 patients with metastatic brain tumor showed definite elevation of CEA levels in plasma and most of them showed values higher than 5.1 ng/ml ranging from 3.0 to 2, 317 ng/ml. CEA in CSF of 55 patients with intracranial tumors were determined. Since CEA levels in CSF obtained from 38 control subjects were mostly 0 ng/ml, ranging from 0 to 0.5 ng/ml, an upper limit of normal CEA level in CSF by one step sandwich method was proposed to be 0.5 ng/ml. Significant elevation of CEA levels in CSF was noted in the one of 37 patients with primary brain tumors, a patient with pineal germinoma showing a spinal metastasis, and in 8 (44%) out of 18 patients with metastatic brain tumor. All of 6 patients with leptomeningeal carcinomatosis showed elevated levels of CEA in CSF. CEA in cystic fluid were measured in 17 patients with intracranial tumors, including 12 gliomas, 2 craniopharyngiomas, 2 metastatic brain tumors and 1 meningioma. Elevated levels of the tumor fluid CEA were noted in 2 patients with craniopharyngioma (8.6 ng/ml and 11.0 ng/ml, respectively) and in one with metastatic brain tumor from breast cancer (14300 ng/ml). Mean value of the tumor fluid CEA of 12 glioma patients was 0.6 ± 0.6 ng/ml. A meningioma and one metastatic tumor showed low levels of CEA less than 0.5 ng/ml in the tumor fluids. Study on sequential determination of CEA in plasma or CSF performed on three patients revealed that CEA levels were well-correlated with the activity of the tumors and could be used for evaluation of treatment or foreknowledge of recurrence of tumors. In conclusion, determination of CEA in plasma or CSF is valuable for differential diagnosis between primary and metastatic brain tumors, evaluation of treatments, or fore-knowledge of recurrence of tumors. Determination of CEA in CSF could be very useful especially for diagnosis of leptomeningeal carcinomatosis.